In the show's 100th episode, Team Flash comes up with a plan to travel into the past to collect items from the Flash's past to build a device to counter Cicada's dagger, despite Barry's reservations. Meanwhile, Sherloque shares his lingering suspicions about Nora to the others.
- All for Nothing: The plan to counter Cicada's dagger fails when Cicada is able to summon enough willpower to call it back from friggin' space.
- Ambiguous Situation: Barry cuts Gideon off as she's saying Nora will be part of a "legion." Would that be the Legion of Superheroes...or the Legion of Doom?
- Ambiguously Evil: It's not clear what Nora's connection to Thawne is.
- And Then What?: Barry's response to Cicada saying he'll kill all the Metas is to ask "What about you?" Cicada says he'll "join" the other Metas when he's done.
- Artistic License – Physics: Cisco's breach to space doesn't create a vacuum.
- Cicada's dagger would have left a crater and created a big impact from being recalled from space. Also, given that it was in deep space and comes back only seconds later, it should've created a sonic boom.
- Back to the Early Installment: Barry and Nora travel to various points throughout the first three seasons, including the pilot, to retrieve the components for a device that the team believes will defeat Cicada.
- Bait-and-Switch: Just before the particle accelerator is activated, Thawne praises Cisco for his work on the project. Then he lifts his hand in the same manner he did when he killed Cisco in an alternate timeline...to shake his hand.
- Berserk Button: Thawne does not take well Barry flatly telling him that no matter what happens, Thawne never gets back to the future.
- Big Damn Heroes: Killer Frost is able to successfully stop Cicada unlike the other members of Team Flash, because her powers didn't come from Dark Matter, meaning his dagger can't depower her.
- Boomerang Bigot: As Barry points out, Cicada is as much of a metahuman as the ones he's killing. He merely shrugs this off and admits that he's planning on committing suicide after he's killed all the others.
- The Bus Came Back: Plenty of characters fitting the 100th episode, including Zoom (with both Tony Todd and Teddy Sears returning), a Time Wraith, Gideon, Grodd (before he became a meta), Harry Wells, Nora and Henry Allen, and of course Eobard Thawne (in his Wells disguise). Hartley Rathaway, The Mardon Brothers, Ronnie, the DeVoes, and Dr. Stein appear via stock footage. Morena Baccarin also recorded new lines as the original Gideon after Legends of Tomorrow opted to use another actress, and is even confirmed to still be around in the present.
- Call-Forward: Before launching the particle accelerator, Thawne tells Cisco that he has a vibe about him and what he's destined to become.
- Continuity Cavalcade: Befitting a 100th episode, a lot of the show's history is touched upon here. This is especially notable during the scene when the Particle Accelerator explosion is revisited, as most of the old footage of events from that night (Stein and his Firestorm matrix getting hit by the dark matter wave, DeVoe and Barry both being hit by lightning, the Mardon brothers' plane being brought down by the wave) is shown again. The mysterious symbols Barry wrote down ("This house is bitchin'") appear in Nora's journal.
- Contrived Coincidence: Barry and Nora happen to land during the events of "Flash Back" when Barry asked for Thawne's help before.
- Crippling Overspecialization: It's revealed Cicada's dagger only nullifies the powers of metahumans if their powers came from Dark Matter. Normally, this wouldn't be a problem, but more metahumans with other sources are coming up.
- Curb-Stomp Battle: It goes back and forth. First, Team Flash counters Cicada's dagger and takes it away from him, with Barry giving him a beatdown afterwards. Then, Cicada summons it back, depowering everybody and getting ready to finish Barry off...and then Killer Frost intervenes, blasting Cicada away and forcing him to retreat.
- Death Seeker: Cicada mentions planning to end his own life after finishing his crusade due to being a metahuman himself.
- Deus ex Machina: Zoom starts following Barry and Nora into the Speed Force gaining on them...and then the Time Wraith that was following them decides it would rather go after Zoom instead; what effect this has on the timeline is unexplored.
- Didn't Think This Through: Team Flash comes up with a fairly convoluted plan to take Cicada's dagger away from him. It works...initially, but unfortunately they didn't put nearly as much thought into how to keep it from him. He summons it back seconds later and goes right back to attacking them.
- "Eureka!" Moment: Nora comes up with the idea to go back to the past to collect the items to build a device to counter Cicada's dagger - a fragment of Savitar's suit, Zoom's Speed Force Transmitter, and Dark Matter particles from the night the Particle Accelerator exploded.
- Evil Has a Bad Sense of Humor: Thawne laughs when he finds out that Barry and Nora want to stop Cicada.
- In the past, he mentioned that he has a "vibe" about Cisco's future, and is clearly stifling a smirk.
- Five-Second Foreshadowing: Cicada only detects four out of the five metahumans on Team Flash present at their confrontation with him before it's revealed Caitlin is unaffected by his power stealing dagger because she's not dark matter empowered.
- Foreshadowing: Nora suggests to Barry while in the past that they get help from Eobard Thawne. Later, we discover that this is not the first time she's done so.
- Futureshadowing: When Barry and Nora go back to the night of the Particle Accelerator explosion, we see Thawne offer his hand to Cisco for a shake, which strongly resembles the way he would later kill Cisco in an alternate dimension.
- Godzilla Threshold: Zoom's Speed Force transmitter is broken, forcing Barry and Nora to turn to Eobard Thawne for help to fix it. Also, from the viewpoint of Thawne, he only helps them because he learns they want to use the device to stop Cicada.
- The Immune: Killer Frost to Cicada's dagger, due to not being infused with Dark Matter.
- Implacable Man: Cicada is outnumbered but does not back down, even summoning enough willpower to call his dagger back from space.
- Internal Reveal:
- Turns out Nora only knew Thawne was her dad's Arch-Enemy - Barry goes on to explain the full details of their feud. She also learns that it was her mom who killed Savitar in the end.
- Cicada also learns that Cisco is still alive.
- Kick the Dog: Another interpretation of Thawne's “At least you have one” comment about Nora is that he is referring to at least he has another family member named Nora alive considering he killed his mother who Nora is named after.
- Know When to Fold 'Em: When he sees that Killer Frost is immune to his powers, Cicada retreats.
- Leeroy Jenkins: Barry wants to go straight after Orlin once they have his identity, but Sherloque points out that as long as he has his dagger, it's a fool's errand.
- Mythology Gag:
- Upon meeting her, Thawne calls Nora "Dawn", which was the name of Barry and Iris' daughter in the comics. Thawne also tells Barry "at least you still have one"; in the comics, Dawn has a twin brother named Don.
- Before that, he also tries to guess who she is. Among the guesses are Jesse Chambers (the comic civilian identity of Jesse Quick), Libby Lawrence and Danica Williams.
- According to Gideon, Nora a.k.a. XS becomes a member of the Legion of Superheroes, much like the comic-book XS did. Moreover, Gideon specifically mentions that it's a 'rebooted' Legion, a sly reference to the fact that XS was introduced in the comics during the first reboot of the Legion of Superheroes franchise.
- Oh, Crap!: Zoom's sudden appearance to stop Barry and Nora. The look he gives Nora is clearly full of hunger for her speed.
- The Reveal: Sherloque's hunch that Nora didn't come to the past on her own accord is proven correct. She came under the advice/instruction of Eobard Thawne - having no idea just how monstrous the man is until she sees him and Barry face-to-face and the utter hatred between the two.
- Ripple-Effect-Proof Memory: A non-human example. It turns out that the 'Speed Force language' that Nora uses to write in her diary ensures that any information recorded using it will remain unchanged despite changes to the timeline. Of course, the fact that Nora needs this suggests that her own memory isn't completely protected from timeline changes.
- Rogues Gallery Showcase: Barry's various enemies, both major and minor, have varying degrees of focus in this episode.
- Shout-Out:
- Ralph plays "Back in Time" when Nora and Barry time travel, much to the team's annoyance.
- Nora jokes about feeling a feeling a disturbance in the Speed Force.
- A Simple Plan: Nora's plan falls apart almost immediately when a Time Wraith begins tailing her and Barry.
- Stable Time Loop: Time-travelling Barry gives Season 2-era Harry the idea to locate Jesse using her frequency.
- Start of Darkness: Dr. Ambres' began when the Particle Accelerator exploded.
- Stealth Pun: Once again, Barry's past nemeses are mentioned in reverse.
- The Stinger: Two of them - one is Nora uploading her journal to Thawne in the future via Gideon. The other is the fall of Earth-90 previously seen in "Supergirl" and "Arrow" to set up Elseworlds (2018).
- Stock Footage: The Stinger from Supergirl is once again reused. However, said stinger was originally made for use solely in this episode before it was decided for use in all three of the participants' respective preceding episodes.
- Strange Minds Think Alike: Both Thawne and Sherloque call Nora "clever girl".
- Talking the Monster to Death: Barry tries to talk down Cicada out of his crusade. It doesn't work.
- This Is Gonna Suck: Barry is still uneasy about using time travel after Flashpoint, but he's finally talked into doing it again, and he really doesn't want to do it. He's proven right when a Time Wraith shows up.Barry: I knew we shouldn't have done this!
- Tranquil Fury: While facing Thawne, Barry says little and keeps a mostly blank face, but his Death Glare and the twitching in his jawline says it all.
- Uncertain Doom: It's not clear what the Time Wraith does to Zoom.
- Villainous Breakdown: Thawne has one when Barry tells him he won't be going back to his time...unless he helps fix the Speed Force transmitter.
- Villainous Rescue: Unintentionally; the Time Wraith chasing Barry and Nora hooks its claws into Zoom while diving in for them.
- Wham Episode: Nora is in the past on somebody else's orders...Eobard Thawne's.
- Wham Line: Thawne addresses Nora as "Dawn", which is likely the name she had in his timeline before he traveled to the past and altered history.
- After Nora tells the prisoner in 2049 that they need to talk, a very familiar voice agrees, "Yes, Nora. Yes, we do."
- Where It All Began: Overlaps with When It All Began due to the time-travel element. Barry and Nora go back to the day Barry's story began - when the Particle Accelerator exploded. At the end of the episode, they also travel back to the night of Nora Allen's death, albeit shortly before the crucial events.
- Write Back to the Future: Using Gideon in the Time Vault as a medium, Nora sends the composition in her journal to Thawne in the future.
- Year Inside, Hour Outside: More like, hours inside, seconds outside, as Barry and Nora return almost immediately from their trip relative to the team.
