Recap / Sherlock S4 E3 "The Final Problem"

Mind the glass...^note Oh, wait, there isn't any.'''

"Today we have to be soldiers, Mycroft. Soldiers."

— John Watson

Sherlock and Watson, who had been shot with a tranquilizer by Eurus, trick Mycroft into acknowledging her existence. Eurus steps up her attacks on Sherlock, culminating in the bombing of his Baker Street apartment. Sherlock, Watson and Mycroft venture forth to Sherrinford, a maximum-security psychiatric facility where Eurus is housed. Although Mycroft is skeptical at the suggestion that she has escaped, the trio discover that Eurus has compromised the staff and controls the entire Sherrinford asylum. She subjects the trio to a series of ordeals, testing their morals by forcing them to choose which of her victims live and die and ultimately forcing Sherlock to confront the memory of "Redbeard", a childhood friend whose death set in motion events that saw Eurus incarcerated. Realising that she will continue to test him until someone he cares for dies, Sherlock tries to connect with her on an emotional level, offering her the love and relationship with a brother that she never had, and Eurus stands down. Sherlock and Watson return to the Baker Street apartment, where they find another message from Mary imploring them to stay together. A time lapse montage shows them rebuilding the Baker Street flat to its original lived-in form before meeting a series of unusual clients.

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Tropes:

• Alas, Poor Villain: Eurus isn't evil or megalomaniacal. She's just tragically, heartbreakingly insane — completely broken by a lifetime of being totally unable to relate to those around her and be related to in turn. All she wanted was a friend. As such she is quickly forgiven by Sherlock and others despite her murdering numerous innocent victims in a sadistic fashion.
• And the Adventure Continues: Unlike the nail-biting cliffhangers of the previous seasons, this season finale ends with no loose threads ^note  aside from the fallout of Sherlock's phone call to Molly, although she's seen happily entering the apartment during the ending montage and Sherlock and John solving crimes from 221B Baker Street like they always have. ^note  Benedict's commitments to the Marvel Cinematic Universe as Doctor Strange (2016) make it very hard for him to have time for other projects. The last shot is of Sherlock and John running towards yet another mystery.
  Mary: When life gets too strange, too impossible, too frightening, there is always one last hope. When all else fails, there are two men sitting arguing in a scruffy flat like they've always been there, and they always will. The best and wisest men I have ever known. My Baker Street boys. Sherlock Holmes and Dr Watson.
• Anguished Declaration of Love: Sherlock and Molly to each other, in a variation. Sherlock says it first when Molly asks him to, and is anguished because he's afraid he'll still fail to save her; Molly is anguished by being forced to admit her love for him out loud since she's afraid Sherlock's just playing a cruel joke on her.
• Arc Words:
  • "Miss me?", "It is what it is", and "Redbeard" are all previous arc words that show up again here.
  • A phrase that is repeated throughout the latter half of the episode: "Today we have to be soldiers."
• Ate His Gun: The governor of Sherrinford shoots himself in the head from under his chin in order to save his wife.
• Batman Gambit:
  • Played against the head of the Sherrinford facility, with the head thinking an old bearded fisherman is Sherlock Holmes and sending the guard in the room with him to Eurus's cell. Turns out the old fisherman is Mycroft and the guard is Sherlock, who now has a key to Eurus's cell. Though, given that the head is serving Eurus, probably a higher Batman Gambit by her on the Holmes brothers is playing out.
  • Indeed, it turns out Eurus has played a ridiculously long and complex Batman Gambit on Sherlock and Mycroft. Since the Moriarty tapes were recorded before his death, and they include one where Moriarty predicts Sherlock would shoot Mycroft instead of John, Eurus must've known years ago that the three men would eventually end up in this scenario. For that to work, she must've known that Moriarty would fail to kill Sherlock, that any other threats on the lives of him, John, or Mycroft would fail, too, that all three would manage to escape the explosion on Baker Street without dying or getting any serious injuries (if one of them had been injured, he couldn't have come to Sherrinford with the other two), that Mycroft would decide the best way to deal with Sherrinford's compromised security was to visit there personally instead of having Eurus moved somewhere else, that the three men would not bring any backup with them, and so on and so on shifting the whole thing into definite Gambit Roulette.
• Big "NO!": Sherlock, John, and Mycroft all give one at once when the governor is about to shoot himself.
• Bittersweet Ending: Tying into And the Adventure Continues, as well: Sherlock and John eventually go back to their lives, strong and able to recover from the Mind Rape Eurus inflicted on them, but Eurus (despite finally understanding her actions thanks to Sherlock) is sent back to Sherrinford due to her actions. However, the ending implies, Sherlock is able to help her recover, as seen with them playing the violin together, eventually with Mycroft and Mr and Mrs Holmes watching at her cell.
• Blatant Lies:
  • Mycroft says early on that Eurus died in a second fire she started, and Sherlock immediately says that he's lying because both he and John had met Eurus earlier without realising at the time who she was. Mycroft admits the lie, and that he told the same lie to their parents. When they find out Eurus is still alive, they give Mycroft one hell of a What the Hell, Hero?.
  • Eurus would have had to be less observant than her brothers, not more, to be unable to tell whether she was having sex with male or female genitalia, even if she only thought to check after, unless she thinks "having sex with" means "firing a gun at".
• Blue-and-Orange Morality: Eurus takes this trope to its logical extreme — she seems to have trouble even understanding the concept of morality.
• Break Them by Talking: Eurus's MO when she isn't killing them. She talks people into helping her if they aren't sufficiently intelligent to be immune (like Mycroft claims to be) or are just inclined to help her anyway (like Jim Moriarty). It's been likened to hypnosis and has resulted in a standing order from Mycroft that no psychiatric evaluation of Eurus must ever happen. One such evaluation was done anyway — it did not end well. This is similar to the "AI-in-a-Box" thought experiment, which suggests that it's impossible to contain a sufficiently super-intelligent AI for precisely this reason — it would be smart enough to suborn anyone it could even talk to.
• Brick Joke:
  • Sherlock finally gets Lestrade's first name right. Lestrade actually does a Double Take.
  • There have been numerous references to the fact that young Sherlock wanted to be a pirate when he grew up. In this episode he finally gets to be one when he hijacks a boat.
    Sherlock Holmes: Sherlock Holmes.
    Boat hand: The detective?
    Sherlock Holmes: The pirate.
• Brief Accent Imitation: Moriarty's pre-recorded videos consist heavily of these.
• Bus Crash: Of a sort. Fans have long been convinced Jim Moriarty would make a comeback and must have been whooping with delight when the prison was taken over by Eurus and he arrives by helicopter. Then we find out this is a flashback to a conversation five years ago.
• Call-Back:
  • This would be the second time someone has thought an old bearded man was Sherlock in disguise. It happened before in The Empty Hearse, where the old man was in fact an old man (much to John's embarrassment). Here, it is a disguise but worn by Mycroft.
  • Way back in "A Study in Pink", Greg said Sherlock Holmes was a great man and that one day if everyone was lucky he'd be a good one. When an officer remarks that Sherlock is a great man, Greg corrects him that he's more than that: he's a good one.
  • Mycroft telling Sherlock that their parents' attempts to let them play with other children didn't work out takes on a whole new meaning now that we know about poor Victor.
  • The Holmes' Uncle Rudi was mentioned earlier in the series by Mycroft in "His Last Vow".
  • In "The Great Game", 221B was blown up early on, but it was caused by a gas leak. It was also the episode in which Sherlock first met Moriarty. 221B is blown up again in this episode, but this time by a grenade on a drone. It's in this episode that Sherlock meets Eurus. Well, meets her again.
  • Mycroft sarcastically tells the prison governor that his security would be better off with clown outfits. A man in a clown outfit earlier attacked him in his home as part of Sherlock's plan to get him to reveal the existence of Eurus.
• The Cameo: Paul Weller appears as the Viking in Sherlock and John's flat.
• Cerebus Retcon: Mycroft previously mentioned that he and Sherlock being introduced to other children was a disaster — it was implied that this was because they found out just how stupid other children are. While they no doubt did find that out, the fact Eurus abducted and drowned Sherlock's playmate made it even more of a disaster.
• Chekhov's Gun: Those nonsensical gravestones on the Musgrave estate that Mycroft mentioned near the start are used to decipher the rhyme Eurus gave Sherlock when they were children, so that he can save John.
• Chekhov's Skill: The crux of "The Six Thatchers" was Sherlock remembering the Latin translation of one of the Arc Words. Another Latin translation is the clue that finally prompts him to solve his first (and the episode's final) case.
• Cliffhanger Copout: Watson being shot at the end of the last episode was just tranquillizer, and then he was left alone.
• Crazy-Prepared: Eurus spent years putting together the pieces before directly confronting her brothers and John, even having contingencies in place in the event any of the participants decided to be clever and go off-script. She even had Moriarty record large amounts of video and audio in order to create the illusion that he still lived.
• Creepy Child: In flashbacks, Eurus is shown being asked why she was cutting her skin (she wanted to see how her muscles worked), making numerous drawings depicting her intent to harm Sherlock, and, ultimately, it is revealed that when Sherlock played pirates with his best friend Victor instead of Eurus, Eurus murdered Victor out of jealousy.
• Criminal Mind Games: How Eurus tortures Sherlock, Mycroft, and John, calling them "experiments":
  • Tries to force either John or Mycroft to murder a man who, though not totally innocent, doesn't deserve to die, or else she will kill the man's wife. When neither of them can do so, he kills himself, and Eurus then murders the wife in front of all of them because they didn't meet the conditions.
  • Forces Sherlock to find a murderer out of a group of three brothers and condemn the guilty man to death. When he does, she kills the two innocent men instead and then follows up by also killing the guilty man, in order to show them that killing innocent or guilty people makes no difference to her at all.
  • Makes Sherlock emotionally torment Molly under the threat of Molly's death if Sherlock fails. After he succeeds, Eurus tells him that Molly was never actually in danger, so Sherlock just manipulated her feelings for nothing. Sherlock does not take this well at all.
  • Tries to force Sherlock into the Sadistic Choice of killing either his brother or his best friend.
  • Leaves John in a Drowning Pit to incentivize Sherlock into solving her puzzle.
  • Makes the men cooperate in all of these things by only letting them talk to "the little girl on the plane", which is supposedly going to crash into a city and kill a lot of people, every time they solve a puzzle. This keeps them compliant despite the emotional torture due to putting the lives of others before their own; however, in the end, the girl on the plane wasn't even real and was all inside of Eurus's head.
• Cruel to Be Kind: Mycroft belittles John to goad Sherlock to kill him instead.
• Death of a Child: Victor, whom Eurus drowned in the well at Musgrave because she was upset that Sherlock never played with her.
• Deconstructor Fleet: Through Eurus, this episode heavily invokes the deconstruction of tropes — particularly Morality Tropes. Eurus, herself, has a psychotic desire to understand how everything and everyone works while damning the consequences.
• Didn't See That Coming: Eurus didn't expect Sherlock to turn a gun on himself rather than Watson or Mycroft.
• Does Not Like Guns: Mycroft, apparently. He refuses to so much as touch the gun Sherlock tries to give him. (Although, he's happy enough to try to use his gun-sword-stick in the episode opener, so it seems he's more just appalled by the thought of cold-blooded violence.)
• Dramatic Shattering: The kitchen window smashes before the drone enters the flat's front room, interrupting Mycroft's statement that because Eurus has allegedly been in Sherrinford since early childhood, neither Sherlock nor John could have met her.
• Drowning Pit: The well at the Holmeses' Musgrave estates. Eurus killed Victor in it when they were children, and almost kills John in it in the present.
• Easily Forgiven: Sherlock and Mycroft seem to almost immediately forgive Eurus for what she did, including, tormenting, raping and even murdering innocent victims of her own volition, not to mention severe personal traumas suffered by main characters themselves.
• Engineered Public Confession: Eurus forces Sherlock to get Molly to say 'I love you' to him, or else she will bomb Molly's flat in three minutes — and he can't imply that anything is out of the ordinary. Molly thinks that he's making fun of her but eventually says it after she gets Sherlock to say it first.
• Even Evil Can Be Loved: Despite what Eurus is, Sherlock, Mycroft and their parents still love her.
  • She loves them, too. She's just emotionally and pathologically incapable of expressing it in any understandable fashion.
• Exact Words:
  • In the first Sadistic Choice, Eurus tells the trio that the governor is to be shot and killed by either Watson or Mycroft, or else the governor's wife will be shot. After neither can bring themselves to do it, the governor takes the gun and shoots himself. Eurus then proceeds to shoot the wife anyway since he wasn't killed by the people she specified.
  • Eurus said the guilty Garrideb brother would die. She didn't say anything about sparing the other two.
• Expy: Eurus Holmes of Hannibal Lecter. Even Lampshaded by the guards outside her cell.
• Eye Open: The episode opens with a shot of the little girl opening her eyes while on the airplane.
• Family of Choice: Demonstrated in this exchange:
  Mycroft: This is a private matter
  Sherlock: John stays.
  Mycroft: This is family!
  Sherlock: That's! Why! He stays!
• Five-Second Foreshadowing: While Mycroft watches the tapes, John goes out into the governor's balcony and looks down at the rocks, for no explained reason. A few seconds later, we learn that he had realised Sherrinford was compromised. He was looking for an escape route.
• Foreshadowing:
  • When Sherlock and John hijack the fishing boat, Sherlock says he's a "pirate". Turns out he used to play pirates with his best friend as a child.
  • Eurus tells Sherlock that only John or Mycroft can shoot the governor. Neither man can bring himself to do it (Mycroft refuses to even take the gun), so the governor opts to shoot himself. Eurus tells Sherlock he will need the last bullet in the gun for later, and he does, when he has to decide whether to kill John or Mycroft. Eventually Sherlock can't bring himself to shoot either of them and, like the governor, opts to threaten to shoot himself.
  • "If the incarceration of my sister is compromised, you will not [leave this island]," says Mycroft to the governor. It was. He doesn't.
  • In some flashbacks of Eurus as a child, she's playing with a toy plane. It turns out at the end that the girl on the plane was actually just in Eurus's head.
  • Eurus tells Sherlock that Mycroft told her that Sherlock had rewritten his memories, something Sherlock picks up on. We find out later that Sherlock was left so traumatised by the death of his best friend, Victor, that he rewrote the memory as that of losing a pet dog, which he had always wanted but was unable to have because his father was allergic to dogs.
• For Science!: The justification given by the governor for disobeying Mycroft's orders. His psychiatric interest in Eurus's uniqueness ultimately led to her takeover.
• Freak Out: Eurus reveals that her threats against Molly were fake and that Sherlock has therefore just put her through the wringer for nothing ^note though she could have been lying about that. Sherlock does not take it well and smashes the coffin to bits in highly uncharacteristic violence.
• Ghost Butler: In the opening scene a door closes after Mycroft without any visible force. Turns out it's part of Sherlock's prank.
• Glassy Prison: Sherrinford's security. Big "clean, invisible" pane of glass. What Could Possibly Go Wrong?
• Grand Finale: For the moment, anyway.
• Heroic Sacrifice:
  • The governor of Sherrinford kills himself in an attempt to save his wife's life.
  • Mycroft attempts to do this for John when Eurus makes Sherlock choose between John and Mycroft to kill. See Cruel to be Kind above. Sherlock knows what Mycroft is doing, so it actually makes it harder for him to kill Mycroft.
• Heroic Self-Deprecation: When asked where Sherlock should shoot him, Mycroft says that he must have a heart buried somewhere beneath his skin, though he doesn't imagine it will make much of a target.
• I Have Your Wife: A literal example; Eurus has the head of the facility's wife Bound and Gagged, and threatens to kill her unless either John or Mycroft kills the head of the facility.
• I Just Want to Have Friends: Eurus' Freudian Excuse. What she was after as a kid (and ever since) was her brother's attention. The rhyme she gave Sherlock was a coded message to play with her instead of with 'redbeard' Victor, and Sherlock solving it as an adult saves John from drowning.
• Idiot Ball
  • The head of the facility ignored Mycroft's orders that nobody be allowed to talk to Eurus. It ended badly for everyone.
  • Mycroft grabbed it when he allowed his sister to talk to Moriarty unsupervised as a "Christmas present". Mycroft for much of this episode, in fact, compared to his characterization as the smarter of the two brothers. (Even the set-up at Sherrinford in general is partly his fault — he's smart enough that not anticipating that the head of the facility would disregard his orders concerning Eurus is a kind of idiocy, for him.)
  • The entire Holmes family (minus Eurus) had one in back-story. A kid went missing on their estate, and apparently nobody thought to check whether there was a well or other such deep hole in the vicinity that he might have gotten stuck down. Usually that'd be among the first things a search-and-rescue operation would look for.
  • The first Sadistic Choice is riddled with holes which you'd expect the Holmes boys, at least, to spot. (The most obvious of which is that there's no guarantee that Eurus won't kill the hostage even if they follow her instructions, so she has no leverage at all.) Everyone in the room has to be holding the Idiot Ball for this to work.
  • When Sherlock asks Molly to say, "I love you," believing that she needs to do so to save herself from Eurus, Molly explicitly tells him that she can't bring herself to say those words to him. There are two other people in the room with Sherlock, and Eurus did not forbid him from putting either of them on the line. Sherlock doesn't even think of this, and almost runs out of time as a result.
  • The trick with the missing glass only works because Sherlock fails to notice the obvious lack of reflection even from inches away. And without glass, he also should have noticed er, "the scent of a woman" as it were.
• Imminent Danger Clue: John notices that the voice in the security footage of Eurus's psych evaluation belongs to the now ex-governor of Sherrinford, meaning he's under her control, as is the entire prison, and Sherlock, Mycroft, and John are trapped. When John voices this realization, he and Mycroft are immediately taken prisoner.
• Informed Ability: Eurus' ability to Break Them by Talking is repeatedly referenced, somehow being capable of turning highly professional people used to dealing with dangerous criminals into her puppets with ease, but she's never shown to be successful at this on screen — the mind games that the audience actually sees are incredibly heavy-handed, and only make their targets more determined to stop her.
• Interplay of Sex and Violence: When Sherlock is being forced to shoot someone, Eurus plays a clip of Moriarty — still in his stationmaster persona — saying "This is where I get off!". Eurus also claims to have "had sex" with a nurse in a way that left the nurse mutilated beyond recognition.
• Just in Time: Molly's declaration of love for Sherlock happens with two seconds remaining on the clock. Subverted, when Eurus reveals that Molly wasn't in any danger to begin with.
• Kick the Morality Pet: Eurus basically forces Sherlock to do this to Molly by emotionally tormenting her, under threat of Molly's death if Sherlock refuses. Sherlock near-tearfully goes through with it to save her, only to find out that Eurus was bluffing. It's worth noting that, during the entire Mind Rape experience they go through, this is the only thing that causes Sherlock to completely freak out and go into a rage.
• Left the Background Music On: From Moriarty, once again. As he makes his dramatic entrance to Sherrinford, the apropos song "I Want To Break Free" is heard. Moriarty even poses a few times like Freddie Mercury. Then he takes out his ear buds and the song abruptly cuts out.
• Lying to Protect Your Feelings: This is Mycroft's justification for not telling his family that Eurus was still alive. He thought it was better that his parents should think she died in a fire rather then let them know she was alive and only became worse over the years. Sherlock is understanding. Mama Bear Holmes is not.
• Magic Countdown: Eurus gives Sherlock three minutes to make Molly say the magic words. The scene plays out in real time but takes 3:40 min until the clock is up.
• The Main Characters Do Everything: Lestrade is in charge of the police operation at the end, though it would seem unlikely for him to be there away from London.
• Making the Choice for You: The Governor takes things into his own hands and blows his brain out since no one else is willing to do it for him.
• Meaningful Name: One of the names on the fake gravestones is "Nemo Holmes". "Nemo" is Latin for "nobody".
• A Million Is a Statistic: At the end of the story, Eurus is directly responsible for the deaths of at least seven people (Victor, the Garrideb brothers, the head of the facility and his wife, Watson's therapist), possibly more, but who cares, since Sherlock is finally playing with Eurus, and the Holmes family trauma is beginning to heal? The fact that these pointless murders are just as traumatic to the families of the deceased is not even mentioned. Averted in that Mycroft does tell his parents it's why she can't ever be released.
• Monster Clown: One haunts Mycroft at his home in the episode's opening.
• Mythology Gag:
  • The series has never used Holmes' famous Master of Disguise skills from the books. It looks like they finally use it this episode... and it's actually Mycroft in the elaborate disguise. Sherlock basically just resorts to a Wig, Dress, Accent. And it's not even a wig. It's just a hat.
  • A Holmes sibling who sits in a room and people bring them problems. Are we talking about Eurus, or Mycroft from the books?
  • Also, a Holmes sibling notable for having weight problems (and actually shown as such in The Abominable Bride)? This time it is Mycroft, and in the flashbacks he is portrayed by an actor who is on the stocky side.
  • The Holmeses' original home being in Musgrave, and the secret Sherlock must solve there, leads to Eurus calling it "The Musgrave Ritual", which is the name of one of the original stories — a story which, like the climax of the episode, involves Holmes decoding a poem in order to find the place where a man is suffocating (although in the book the man is a treasure hunter and Holmes arrives too late to save him). The poem even contains a few of the same lines in both versions. And it's a story Holmes tells Watson about when he first became a detective, just as Eurus calls Sherlock's attempt to solve the ritual as a child his "first case".
  • One of Eurus's puzzles for Sherlock, in which she forces him to figure out which of the three brothers Garrideb committed a murder, is a reference to "The Three Garridebs".
  • Eurus mentions that Moriarty has a brother who's a stationmaster in a throwaway line, as Holmes himself mentions in "The Valley of Fear."
  • "Rathbone Place" is seen on a sign in the final shot, a nod to Basil Rathbone, who played Sherlock Holmes in multiple films.
  • One of the scenes in the And the Adventure Continues ending has the Dancing Men code from "The Adventure of the Dancing Men" written on a blackboard.
  • Mary's closing monologue evokes the poem 221B by Vincent Starrett, especially when she talks about how there'll always be Holmes and Watson.
    Here dwell together still two men of note
    Who never lived and so can never die:
  • In the same monologue, calling Sherlock and John "The best and wisest men I've ever known" is exactly what Watson calls Holmes at the end of "The Final Problem".
  • The final shot evokes a popular promotional image◊.
  • Mycroft making very stupid mistakes despite his intelligence is a trait he shares with his literary counterpart; in "The Greek Interpreter" Mycroft seriously endangers the client of the case, Mr Melas, by advertising for relevant information in all of the London papers, thus letting the villains of the piece know that Mr Melas has squealed on them, and they almost succeed in killing him in retaliation.
  • Eurus is the Greek goddess of the east wind; the references to the east wind also refer to the ending of "His Last Bow", where Holmes says "There is an east wind coming" as a metaphor for the imminent war.
• Nice Job Breaking It, Hero: Everything happened because Mycroft was thick enough to let his sister talk to Moriarty — unsupervised. That conversation set the stage for the events of this season and plans for Moriarty's revenge.
• Not So Stoic:
  • Mycroft, full stop. Up until now, he's been played as an ice-cold man who only really cares about his job and his brother, but he's the most visibly affected by Eurus's fatal mind-games. (Though this is possibly because it's his sister doing these terrible things.)
  • When Sherlock finds out that Eurus tricked him into manipulating Molly's feelings for him, he grows furious and smashes the coffin to bits, at one point even letting out a scream of rage.
  • Even Eurus is subject to this. She sets up the men's final "experiment" to have Sherlock shoot either Mycroft or John. Sherlock decides "To hell with that" and points the gun at himself. Up to that point, Eurus had been almost robotic and detached from the concept of emotion. As soon as Sherlock makes this decision, however, she audibly loses it, as if throwing a tantrum because he "cheated".
• Obligatory Earpiece Touch: Mycroft talks to Sherlock via earpiece and both characters use the touching gesture while talking.
• Oblivious Janitor Cut: While Sherlock, John and Mycroft have to deal with the lethal drone, the camera pans to Mrs Hudson downstairs vacuuming her floor to loud rock music, completely unaware of what's going on above.
• Oh, Crap!:
  • Mycroft in his first scene when the words "I'm back" flash on the screen over old home video footage of himself and Sherlock as children.
  • Mycroft when John Watson points out to him that the voice on the tape of Eurus talking someone into helping her belongs to the head of the Sherrinford facility. He then captures them.
  • Sherlock gets one when he realises there is no glass separating him from Eurus in her cell, as she's able to interlock fingers with him.
  • Sherlock, John, and Mycroft when Molly is revealed to be Eurus's next target.
  • Eurus, John and Mycroft have one when Sherlock threatens to shoot himself.
• One-Liner, Name... One-Liner: John when reminding Mycroft that they have to save the girl on the plane.
  "Today, we have to be soldiers, Mycroft. Soldiers."
• Outrun the Fireball: We see Sherlock and John jump out of the window to escape the explosion in 221B.
• Pensieve Flashback: Early on we see flashbacks to Sherlock and Mycroft's childhood with them being present as adults.
• Plot Armour:
  • Because no-one really thought that John Watson was going to be shot dead at the end of "The Lying Detective", did they? On the other hand, there was every possibility that Mycroft might have been shot in the room where Eurus asked Sherlock to choose between Watson and Mycroft.
  • All three manage to escape 221B just as a bomb is going off, and later show up without a single scratch.
• The Reveal:
  • The girl on the plane isn't actually real; it's all in Eurus's head.
  • Redbeard was not actually Sherlock's dog. It was his (human) best friend whom he used to play pirates with (and the boy's pirate name was "Redbeard"), and Eurus killed him out of jealousy because she wanted to play, too.
• Sadistic Choice: The modus operandi for Eurus's emotional torture of her brothers and John. She puts them in contact with a plane that's likely to soon crash (which would probably cause the deaths of many innocent people), and forces them to play along with her and do terrible things in order to be put back in contact with the girl on the plane so they can save her and other innocent lives. Eurus's individual puzzles qualify, too:
  • First, she demands that either Mycroft or John kill the head of security at Sherrinford, or else she will murder the man's wife. Ultimately, neither man can bring himself to do so, leading the head of the facility to kill himself instead; however, this proves to be all for nought, since Eurus kills the wife anyway since Team Sherlock didn't properly follow her demands.
  • Second, she has Sherlock solve which of three brothers committed a murder, telling them that the guilty brother will be dropped into the sea.^note Since all of them are bound and gagged, they're almost sure to drown. Sherlock answers correctly, knowing that he's condemning a man to die either way... and then Eurus drops the two innocent brothers instead. When John calls her out on this, she responds by... also dropping the guilty brother.
  • Next, she tells Sherlock that Molly Hooper has a bomb hidden in her house that will detonate in three minutes unless Sherlock gets Molly to admit she loves him (essentially toying with her feelings purposely in the process). Sherlock does it to save her, and Molly says the words... only for Eurus to admit that the bomb was a fake and Sherlock just played with Molly's feelings for nothing. This leads to a Freak Out on Sherlock's part.
  • Finally, she tries to force Sherlock into the hardest choice of all — killing either his older brother or his best friend/Living Emotional Crutch. Sherlock is able to Take a Third Option to get out of this, though.
• Schmuck Bait: Mycroft left explicit orders that no psychological evaluation of Eurus was to be done by the Sherrinford staff. The head of the facility and the staff psychologist found the opportunity to interview her too tempting to resist. It ended very badly for both of them.
• Senseless Sacrifice: The prison governor Ate His Gun to save his wife—which was against the rules set up by Eurus, so the wife gets shot anyway.
• Sequencing Deception: The episode briefly makes it look like Moriarty is still alive by having him show up at Sherrinford in a helicopter. It's only once he's gone inside, and is face-to-face with Mycroft, that the "Christmas Day, 5 Years Ago" text appears to confirm that it's a flashback.
• Shout-Out:
  • With both Moffat and Gatiss being Doctor Who fans turned writers, the set-up of a supervillain apparently imprisoned in an island fortress and managing to suborn the commander is almost certainly a reference to the Master's situation in the story "The Sea Devils".
  • "Do you trust your wife?"
• Status Quo Is God: At the end, the blast-damaged 221B living room is rebuilt... to be exactly the same as it was before. Including the same wallpaper, spray-painted smiley face, bullet holes, and letter with a knife through it on the mantelpiece.
• Stress Vomit: Mycroft vomits after seeing the governor blowing his head off.
• Stupid Good: Zig-zagged. One take on John's refusal to shoot the head of the facility when the life of the man's wife was at stake. By not getting blood on his hands there were two deaths (the head of the facility and his wife) instead of one. Of course, this assumes that Eurus wouldn't have killed her anyway, and perhaps the next scene (where two innocents are dropped to their deaths) is meant to show this to be the case (in which case, it makes everyone even more Stupid Good, as they considered killing someone because an untrustworthy sociopath told them that she might not kill someone else if they did so...)
• Surprisingly Realistic Outcome: Sherlock's parents are pissed at Mycroft for lying to them about their daughter being alive, and not dead like he told them. It doesn't matter that she was responsible for many unfortunate events prior to the episode, they had a right to know if their daughter was dead or alive and the fact that Mycroft didn't bother telling them this most likely damages his relationship with them.
• Sword Cane: We learn that Mycroft has a sword in his trusty umbrella... which also carries a pistol, when he removes the blade from the handle.
• Take a Third Option: Sherlock, when Eurus tries to force him to kill either John or Mycroft. Sherlock instead opts to kill himself, which causes Eurus to have a Villainous Breakdown by losing her cool for the first time in the episode and tranquillizing him to keep him from going through with it.
• Tap on the Head: John is knocked unconscious for a significant period of time but wakes up and is instantly fine, rather than brain-damaged.
• Tin Man: Mycroft expresses doubt that he has a heart, and suggests it's probably quite small in any case, after acting like a heartless jerk to spare Sherlock's feelings and trying to sacrifice himself for John's sake.
• Title Drop: At the end of the opening scene, Moriarty says to the girl on the phone: "Hello, my name's Jim Moriarty. Welcome to the final problem!" The same line is heard again as a recording when Sherlock, John, Mycroft, and David (the governor) are all in Eurus's cell at Sherrinford. Later, Eurus tells Sherlock he has to solve "the Musgrave ritual, his first case, and the final problem", as the well with John in it starts filling with water.
• Tranquillizer Dart: Eurus' weapon of choice. John is said in the opening to have been taken out by one . Later, Sherlock is knocked out by one to prevent him from killing himself.
• Trash the Set: 221B is destroyed part-way through the episode thanks to a bomb. It is, however, rebuilt during the montage at the end.
• Trauma-Induced Amnesia: We learn that Sherlock blocked out the true meaning of "Redbeard" since he couldn't bear the Awful Truth.
• Villainous Rescue:
  • In the last of Eurus's games, Eurus shoots Sherlock with a tranquillizer to stop him from shooting himself.
  • At the end, after Sherlock finally solves Eurus's puzzle, Sherlock convinces Eurus to save John from the well.
• Wham Line:
  • From Eurus near the end of the episode, accompanied by a Wham Shot:
    "Remember Daddy's allergy... what was he allergic to? What he would never let you have, no matter how much you begged? Well, he never let you have a dog. Funny little memory, Sherlock. You were upset, so you told yourself a better story. But we never had a dog."
  • Another occurs when Sherlock finally figures out Eurus's motivations via a song that she used to sing to him a lot of times in his childhood. It contains a secret message when certain words are extracted with the help of misleading gravestone dates.
    I am lost. Help me, brother.
    Save my life before my doom.
    I am lost without your love.
    Save my soul. Seek my room.
• What the Hell, Hero?:
  • Sherlock gets into Tranquil Fury at Mycroft for letting Eurus talk to Jim Moriarty.
    Sherlock: Five minutes. It took her just five minutes to do all of this to us.
  • There's an even more explicit one after the governor has killed himself and Sherlock, Mycroft, and John are being called into another room, with the gun in Sherlock's hands.
    Mycroft: She's very clever.
    Sherlock: I am starting to think that you are not.
  • Mr and Mrs Holmes tear into Mycroft for hiding the fact that Eurus was still alive all these years.
• Wig, Dress, Accent: Mycroft's convincing disguise as a sea dog.
• Window Love: With a twist. Eurus in her Glassy Prison raises her hand to touch Sherlock standing outside and he follows suit only to realize that there is no glass and both hands interlock.
• The Worf Effect: Sherlock and Mycroft, two of the smartest characters in the series, are repeatedly outsmarted and manipulated by their own sister, Eurus to demonstrate the antagonist's incredible intelligence and cunning. Mycroft in particular practically becomes a Butt-Monkey to emphasize how much he was out of his depth.
• You Wake Up in a Room: After Sherlock is knocked out via tranquillizer dart, he wakes up in an unfamiliar room which turns out to be just outside the family's homestead, far away from Sherrinford. Also, John doesn't know how he got into the well.

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