The seventh game in the Professor Layton series and the first main game not to feature Professor Layton as the protagonist. Instead, it follows his daughter Katrielle Layton, who works as a private detective solving mysteries around London.
The game was released worldwide on iOS and Android on the 20th of July, 2017, with the 3DS version being released in Japan on the same date. The 3DS version was released internationally on October 6, 2017. The Nintendo Switch "Deluxe Edition" version was released in Japan on August 9th, 2018, and in North America on November 8, 2019. A "Deluxe Edition Plus" was also released in Japan on July 8th, 2021, with the option to switch between the original voice actors for the game and the ones in the anime.
It received an aforementioned anime adaptation as Layton Mystery Detective Agency: Kat's Mystery-Solving Files in April 2018.
Layton's Mystery Journey: Katrielle and the Millionaire's Conspiracy contains examples of:
- Aborted Arc: The prologue of the game introduces Sherl, who doesn't know who or what he is and he asks Kat to help him out. This is abandoned as soon as the first case starts. Ultimately, it doesn't get brought up again until one of the post-game Case Codas, at which point it is interrupted by Hastings bringing another case.
- Alertness Blink: The ! to indicate a puzzle now turns into a ?.
- And Your Reward Is Clothes: In the 3DS release, players can discover Fashion Farthings, which can then be exchanged for one of Katrielle's special outfits, barring paid-DLC exclusives.
- And Your Reward Is Interior Decorating: For every five puzzles you solve, you get a new option for decorating the Layton Detective Agency.
- Anti-Frustration Features: This game not only gives you the ability to go back to cases you've already solved in order to find puzzles and hint coins you may have missed, but it also tells you precisely how many puzzles and hint coins are in each area, along with how many you've found. Anyone who has ever experienced the nightmare of running all over the entire map going on a mad Pixel Hunt to find the solitary puzzle they missed will welcome this feature with a loving embrace.
- Batman Parody: One case involves Katrielle investigating the disappearance of a superhero named Ratman, who was mostly based on the '60s version (but some '90s movies references, like "Ratman Forever", were made as well).
- Big Bad: During most of the game, there doesn't seem to be a main antagonist (aside from some thieves in Chapter 8 who are arrested due to a robbery attempt that was resolved in that same chapter), as Katrielle is instead dealing with personal troubles affecting respectively the Seven Dragons. However, it turns out that all seven of them also share a much bigger problem that is being exploited by Lord Adamas (who is none other than Ernest Greeves), who is looked for and confronted in the final chapter.
- Big "WHAT?!": Said by Ernest and Sherl after Kat says the Wham Line in a post-credits scene.
- Catchphrase: Katrielle has "Puzzles are made to be solved!" and "This mystery is history!" She also uses a gender-inverted version of one of her father's phrases; "That's what a gentlewoman does." (Or sometimes "That's not what...", based on the situation.)
- Clear My Name: Case 11: "Katrielle Layton: WANTED!" has Katrielle accused of murder by Chief Inspector Royall Britannias. The "murder victim," Ms. Pryce, had died from a fatal blow to the head by the mannequin head she was trying to fetch down from a cabinet when gravity caused it to strike her without aid from anyone else. Only after he failed to revive Ms. Pryce did the Chief Inspector, then driven by Envy, concoct a scheme to frame Katrielle for murdering Ms. Pryce, and discredit Inspector Hastings, who had benefited from Katrielle's puzzle solving by painting HIM "who solicits the help of criminals at the same time." As Hastings HAD been helped by Katrielle, the Chief Inspector had been "losing ground" and feared if his job was taken from him, that he'd be a failure again (which given he was born with nothing and losing what he'd had now was his greatest fear).
- Clueless Mystery: Case 12 hides the most damning evidence that Ernest is actually Miles Richmond, and thus in turn Lord Adamas, until after the big reveal a short while after Katrielle first discovers it. Somewhat justified given how obvious it'd be if the player did get to see it for themselves, therefore ruining the dramatic reveal. The case withholding other vital information from the player, like the documents Katrielle discovers, and the information she had gotten her allies at Scotland Yard to uncover for her off screen, are more up for debate though.
- Continuity Nod:
- The letter Katrielle sends at the start of the game is addressed to Lucy.
- When Sherl first presents his case to Katrielle, she asks if there's something more to it than a mere talking dog, like a deadly box, a letter from the future, or relics from an ancient civilization; all of which were things her father dealt with in his games.
- Dean Delmona appears with white hair when it was a plot point in Professor Layton and the Unwound Future that he wears a wig. His animation reveals that this too is a wig — it jumps straight off his head when he is surprised — so he probably just realized that once his facial hair went gray, he couldn't pull off the brown wig anymore.
- Stachenscarfen appears throughout the game, offering puzzles and references to previous Layton titles.
- Granny Riddleton likewise returns, though she's no longer the keeper of lost puzzles. She's lost a lot of her eccentric traits as a result, becoming a generic Granny Classic and a bit of a gossip-monger.
- Another returning NPC is Benny, the popoño seller from Azran Legacy; having traveled to London to try and market his popoños internationally.
- During the scene where Katrielle writes to Lucy during the final Coda Case, she mentions her brother Alfendi.
- Demoted to Extra: Granny Riddleton, who had already lessened her presence in the prequel trilogy due to continuity reasons, has her presence further diminished in this game, whose gameplay format and structure make it so none of the puzzles are missable anymore. Thus, Granny is now an ordinary NPC who only appears in some chapters.
- Didn't Think This Through: By giving Ernest a focus to survive, his dying mother hoped it would give him strength to face life alone. But since it was based on revenge on those who ruined them—the Seven Dragons—she didn't know he couldn't get past it and see the folly of it, as Katrielle points out. The loss of his grandfather and mother was enough to make Ernest intent on fulfilling the promise.
- Disappointed by the Motive: Katrielle gives this to the Big Bad, Lord Adamas or actually her assistant, Ernest Greeves, when she realizes that Adamas was plotting to financially ruin the Seven Dragons in revenge for them supposedly doing the same thing to his grandfather. She's saddened that he would put all his talents toward a goal like that, especially since he'd helped her all game. There's a bit more to the story than that, but by the end of the night, Adamas agrees with Katrielle.
- The Dog Was the Mastermind: The shadowy figure behind the titular conspiracy is none other than Ernest, the absent-minded college freshman and bumbling new hire at Katrielle's detective agency who follows her along to every case as her sidekick. That being said, he doesn't oppose Katrielle at all and in fact is in full support of her; Ernest just wanted to gather all the millionaires together in one place so he could ruin them out of revenge.
- Downloadable Content: Mystery Journey stands out for being only Professor Layton game to date with paid DLC. Even then, they're just a few non-essential puzzles and extra clothes for Katrielle; the Daily Puzzles are still on the house, like all previous games.
- Easily Forgiven: Almost every culprit is this. Mostly justified that they are redeemable, although how justified it may varies from case to case based on any particular player. The closest the cases come to an exception is case 08, in which Captain Midas and co. are still arrested after attempting to steal a golden statue, and case 11, where Chief Inspector Britannias tried to make someone's death out to be a murder to frame Katrielle as their killer. They're the only times when the case culprits are actually arrested for their actions, although in each they are still made to be extremely sympathetic. In the case of the former Mustafa Fulhold forgives Midas and offers him a dream job for after he's served his punishment, and in the latter, Emiliana empathizes (in one of the case codas) how Britannias isn't a bad person and deserves a lot of sympathy.
- Endgame+: Every case (including the last one) has its own version of this. You can go back to the detective agency and return to solved cases at any time to look for puzzles, Hint Coins, or collectibles you missed (each screen even gains a counter showing how many puzzles or Hint Coins are still remaining to find). The story sets itself to just before you solved the case, so you can watch the case's ending again if you want.
- Establishing Character Moment: Kat is just barely done hanging her sign when Sherl calls out to her. Her response to a talking dog? Completely nonchalant!
- Fatal Flaw: The Chief Inspector's envy towards the suddenly improving Inspector Hastings made him concerned he might lose out to Hastings. When he sees the victim of an accident involving a mannequin head, after an unsuccessful attempt to save her, decides to use this as a way to frame Katrielle for her murder and discredit Hastings at the same time.
- Foil: Ernest can be seen as one towards Clive, AKA Future Luke from Professor Layton and the Unwound Future. Both lost their families at a young age and became bent on revenge, but while Clive started off with little money and was adopted into a rich household, Ernest AKA Miles was born into a rich household that became destitute. Clive used his left fortune to build a grandscale "future London" under the real London as part of his grandiose revenge scheme, while Ernest simply used his old family home as it was, utilising rumours about it, and only adding in an Eternal Diva-esque puzzle party. Clive impersonated the titular character's assistant, while Ernest actually is an (albeit initially unwanted) assistant. Also, Clive in the end was still largely alone and likely went to prison, while no charges were pressed against Ernest and the Seven Dragons, the people whom he wanted revenge against, welcomed him back. The biggest difference between Ernest and Clive is that Ernest is almost entirely none-subjectively a likeable and sympathetic character, which is helped largely by his extremely focused revenge scheme that would've resulted in damage only to those he has beef with. Whereas people's sympathy towards Clive tends to vary far more, given his scheme for revenge was a lot more diabolical and specifically targeted innocent people. This is also why no charges were pressed against Ernest, who at most committed a few crimes like coercion, blackmail, and possibly even a few more serious charges, if you really wanna stretch what Ernest did, whereas charges were pressed against Clive because he literally committed a large scale terrorist attack on London.
- Frictionless Ice: There's a series of Daily Puzzles where you have to form squared or rectangular groups of penguins by making them slide towards each other in the icy disco stage.
- "Friends" Rent Control: Katrielle Layton not only has an apartment somewhere in London, which she apparently lives in by herself, she also has an office for her detective agency somewhere else in London within walking distance of Big Ben, even though her only apparent source of income is her newly created detective agency which is described many times as not getting much business. Although given her father's fame and connections, it's possible Hershel pulled some strings to ensure she'd be financially secure even in his absence.
- Have a Gay Old Time: Ernest occasionally speaks with these, but Kat has also referred to something peculiar as "queer".
- He Went That Way: The Lipski brothers when Kat is fleeing the police after a false murder accusation.
- 100% Heroism Rating: Over the course of the game, Katrielle builds up quite the reputation for her detective work. So much so that when she is accused of murder in Case 11, while there were a few who were initially shocked by the accusation, absolutely no one other than the accuser believed that she did it.
- Hurricane of Puns: Any time Sherl gets on a roll with his dog puns. He calls criminals "mongrels" and young people "pups". He also refers to Kat as a...well, cat.
- Kat also has a penchant for wordplay, especially when solving mysteries.
- Notably, every new character (and one returning one) in this game has a Punny Name. Kat would seem to be the odd one out, but her name is repurposed into enough puns on its own to count.
- Is This a Joke?: Done a few times leading to My God, You Are Serious! when "It is" is the reply to "Say it isn't so!"
- Killed Off for Real: Averted with the couple who died at the Thames. It was all staged by Pipper in order to attract people to the rather unpopular Riverside Festival she's so fond of. However, Case 11's victim plays it straight, although it was an accident.
- Laser-Guided Amnesia: Sherl cannot remember who he is, where he came from, or why he can speak.
- Later-Installment Weirdness: This having been the last Layton game for a long time (until the announcement of New World of Steam in 2023), many of the changes seen in it are to be expected given the different protagonist (Katrielle Layton, instead of her father), who employs a less rigorous (but still effective) methodology to solve cases (as that's what she does instead of investigating one huge, overarching case), and interacts with a different cast of supporting characters. But there's one major novelty that calls for attention: Whereas the previous games progressed as the player completed the chapters one by one, in this game multiple chapters will be unlocked at once at one point, and they can be completed in any order — this was likely inherited from the open-ended concept of the fourth chapter of Azran Legacy.
- Lighter and Softer: Compared to her father's series, Katrielle's cases are less violence-involved. Even the last two chapters, which deal respectively with Katrielle being accused of murdering someone and Lord Adamas wanting to kill the Seven Dragons, have their darkest aspects toned down.
- Love at First Sight: Ernest Greeves, Katrielle's assistant in the game, fell for her the moment he first set his eyes on her when they first met sometime before the events of the game. Her proving his innocence when he was accused by Ercule during said first meeting combined with her more than impressive detective skills only made him fall for her even harder.
- Metapuzzle: The special minigame Ideal Meal operates in a similar manner to Dress Up from Azran Legacy. Over the course of the game, Katrielle and her friends obtain culinary dishes by solving puzzles. Then, in the minigame, she has to make use of these dishes to give the customers their ideal dinners. And since all dishes are usable for each of the eight customers and each dish can only be assigned to one customer at a time, the player has to work around the clues and lore provided by the customers and their acquaintances in order to ultimately satisfy all of them. It's the only way to fully win the minigame and unlock additional content in the postgame.
- Moon Logic Puzzle: There's a number of these, more so then is even usual for the Layton series. One of the puzzles gives the player a brief explanation on the pH scale, and describes that pH 3 indicates acid and pH 7 indicates alkaline. It then asks what pH 0+0 would indicate. The answer is "photo". Because pH 0+0 kinda looks like "photo".note Another puzzle has a man talking to his wife about how last year it was their seventh wedding anniversary on the 30th of June, while next year it'll be their tenth wedding anniversary. You have to say when the conversation was taking place. The conversation started on the 31st of December at 11:59 at night and halfway through the sentence it became midnight and the date changed to the 1st of January. Meaning the start of the sentence took place in the year of their 8th anniversary, while halfway through, it became their 9th. Just forgetting about the fact that it's pretty awkward that you have to put the date that the conversation specifically started, you don't exactly expect people to change the context of what they're saying based on a to-the-second clock, mid-conversation.
- Mythology Gag:
- The final case has a "Puzzle Party" that functions just like the one in Professor Layton and the Eternal Diva.
- The first time you visit Madame Doublee's house, you can find cat statues and plush toys referencing puzzles and the cat from Unwound Future.
- Several of the collectible trinkets you can find reference previous games: The "Magnificent Moustache" refers to "slicked-back hair and a pink shirt" in a reference to Aldus from the prequel games. There's a popoño from San Grio and a snow globe from Froenborg in Azran Legacy. A music box appears to have figures of Anton and Sophia from Diabolical Box. The "Lovable Lake-Dweller" plushie is based on Last Specter. And one last item is a scale model Laytonmobile.
- The Nicknamer: Sherl has nicknames for everyone, almost never using anyone's actual name.
- Nostalgia Level: Though London also appeared in Azran Legacy in a limited capacity, Mystery Journey is the first game since Unwound Future where the city serves as the primary setting of the adventure. This includes the reappearance of major locations that weren't even playable in Azran Legacy, like River Thames.
- Old Save Bonus: Not necessarily old, but those who have participated in the Real Puzzle Solving ARG can transfer acquired items into their copies of this game.
- Opening the Sandbox: After Katrielle manages to solve her first four cases, she receives the following four at the same time, allowing her and her friends Ernest and Sherl to tackle them in any order.
- The Promise: In the final case: Miles promised his mother he'd be big and strong and as important as the people who stole the diamonds from the Richmond family. He then became Ernest and later Lord Adamas.
- Reality Is Unrealistic: One of Katrielle's catchphrases is "the truth is stranger than fiction", and her crime-solving abilities are considered miraculous for the fact that she actually realizes these bizarre truths when other people wouldn't even consider them. Truths that Katrielle arrive at include Big Ben's hour hand being made of wafer, a bird being the culprit because it ate too much and did a massive poo, and an apparent "bank heist" actually being a freak accident caused by a malfunctioning ventilation. And she's always right.
- Red Herring: There's quite a few of these across the cases to throw you off the truth.
- Case 01 has the clue you gain about the recent span of metal thefts, which turns out to be totally unrelated to Big Ben's missing clock hand. Likewise, the missing cutlery is also unrelated and is later explained in a case coda as a happenstance incident.
- Case 03 leads you down the path of believing the culprit to be either Maverick D. Rector, and present a strong motive for Eddie Torrie. Neither are the culprit.
- In Case 04 when the agency crew are directed to a photo of Rex, Madame Doublée's pet they need to hunt down, by he maid. Since she doesn't bother clarifying, they think she's directing them to the large photo of the big white cat, but it's actually the smaller one of the igunana underneath.note Rex is also a very dog like name, to mislead players into believing that Rex may actually be a dog, which is actually an assumption Sherl makes.
- Case 06 has a character called Shadee who's hanging around a bank acting, well, shady after a supposed bank heist. Katrielle and company assume him to be related to the case and pursue him. Turns out he isn't related at all, and wasn't even running away from them on purpose.
- Douglas Dert seems like a prime candidate for Ratman's true identity in case 07: he's a reporter for the London Times who seems to pop up everywhere you go, and is always the one writing articles on Ratman. It's a pretty familiar set up to anyone familiar with superheroes, like Superman and Spider-Man. There's even a few times where he'll begin to rhythm, which is one of Ratman's traits. He isn't Ratman, but someone else connected to the London Times is.
- In case 11 Katrielle & Sherl are almost crushed after the giant ball of the Bowlyn statue comes loose just after they reach the resolve to undercover the truth of Ms. Pryce's murder. They speculate, and quickly discard, that it may have been the police's doingnote , whereas players are likely to assume it was the culprit. Turns out it was no one's doing, it just happened to come loose. The actual culprit is one too, as he is made out through the case like he is an obvious villainous culprit. While he did indeed try and disguise the Ms. Pryce's death as a murder to frame Katrielle, he didn't kill her, and actually tried to help her.
- Ripped from the Headlines: The end of the first case strongly implies that the point of the meeting with the ambassador was to help Britain stay in the rest of Europe's good books during Brexit, Britain's exit from the European Union.
- Rule of Seven: The eponymous millionaires are a group of seven wealthy individuals known collectively as the Seven Dragons. Each of them is involved in one of the cases Katrielle investigates (from the second to the eighth), though they're all good characters. They do hide a dark secret, though it's more of a major regret they share, and isn't revealed until the final chapter.
- Secret Identity Vocal Shift: Cesar Chance's voice is quite nasally normally, but as Ratman, he has a baritone. Shown in most adorable fashion when he's taking care of his newborn baby.Chance: Make way, make way! Here comes (turns on the baritone) Ratman, to save the day.
- Separated by a Common Language: Ernest is most guilty of this, sometimes to the point you'd need a base knowledge of British slang to understand him all the time, or else a knack for context clues.
- Sequel Hook: The ending suggests that Katrielle and her friends are hard at work on their unsolved mysteries of Hershel Layton's whereabouts and Sherl's true identity, then adds another twist- Katrielle is not Hershel's biological daughter.
- Shout-Out:
- For starters, the dog's full name is Sherl O. C. Kholmes.
- Inspector Ercule Hastings is one to Poirot: His name is a combination of Hercule Poirot and Captain Hastings; he speaks like Inspector Japp and he and Katrielle have a similar relationship to Japp and Poirot; his wife is named Felicity and he calls her his 'little lemon', Felicity Lemon being Poirot's hyper-efficient secretary.
- Kat's cases involve a group of British billionaires called Dragons. The last case gives the Dragons the chance to use one of the show's catchphrases, "I'm out!"
- Considering that Case 10's prime suspect is Ernest, it's only fitting that the title pay tribute to one of Oscar Wilde's most beloved plays. And the allusion does not end there: like in the play, Ernest isn't his real name.
- "Oh yes. Wafer makes perfect sense... for a kitty Kat
!" - Sometimes when Sherl solves a puzzle, he sings "I ain't nothin' but a hound dog!"
- One of the collectibles to find is a golden idol, with a warning that a giant boulder might crush you if you touch it; clearly referring to Raiders of the Lost Ark.
- One collectible is a yellow eye mask based on Yo-kai Watch.
- An item called the Bubble Blaster is very obviously based on the Oxygen Destroyer from the original Godzilla (1954).
- A statue shaped like The Incredible Hulk can be found at the wharf in Case 4.
- The entirety of Case 07 is centered around references to the Batman Film Series. The case itself is titled "Ratman Returns", the Case Coda that's unlocked by beating it is called "Ratman Forever" (which is also shouted by a character during the case), and several times throughout Ratman and other characters sing some variation of "Da na nana-nana nana RATMAN!"
- Spin-Offspring: Katrielle, like her brother Alfendi in Layton Brothers: Mystery Room.
- Suspiciously Similar Substitute: Ernest Greeves from takes many cues from Luke Triton (being the protagonist's assistant and biggest admirer). Ironically, in the game's last chapter, he acts closer to Clive, Luke's "future" impersonator in Unwound Future.
- Take Your Time: No matter how urgent the case is, you can still find hidden puzzles and Hint Coins. Even when you're fleeing the scene from a false murder accusation with the police hot on your heels.
- Trailers Always Lie: All the promotional material for the game suggests that there will be an overarching arc of Katrielle searching for her father, while his disappearance is really only mentioned in a few cutscenes and Kat is only seen searching for him in the intro.
- Technician Versus Performer: Emiliana Perfetti is a criminal profiler and relies on facts and hard evidence to make her deductions. Katrielle is a private detective who follows hunches and gut feelings to find the truth.
- Title Drop Chapter: The game's Spanish version adapts the game's subtitle for the 12th and final chapter (minus Katrielle's name). The English version averts the trope by naming the chapter "Diamonds Aren't Forever", though it does serve as both a Shout-Out to a song by Bring Me The Horizon and a pun on the phrase when referring to the diamonds that made the Seven Dragons wealthy.
- Toggling Setpiece Puzzle: One of the main minigames you can play revolves around Sherl escaping from the dog pound where he's captive. The exit is in plain sight, but along the way there are several color-coded plates that are obstructing Sherl; for each colored group of plates, there's a button that swaps their states (lowering the active ones while elevating the inactive ones. Because Sherl cannot stop moving in each turn until hitting a wall or an elevated plate, the player has to think carefully about which colored plates to keep active or inactive, and when to swap their states. And in each level of the minigame, Sherl has a limited number of turns to reach the exit.
- Unusually Uninteresting Sight: Sherl, a talking dog, doesn't quite strike Katrielle as something she should be completely awed over or fascinated about because to her, Sherl is just a talking dog and nothing more.
- Updated Re-release: Originally released on iOS and the Nintendo 3DS in 2017, the game received an enhanced port for the Nintendo Switch in 2018 in Japan (and 2019 overseas), updating the gameplay features to take advantage of the system as well as adding a large number of puzzles to the already robust collection. It also adds the option to switch between the original voice actors for the game and the ones in the anime.
- The Very Definitely Final Dungeon: The setting of Case 12, which is itself the final chapter in the game, takes place in the mansion of the late Maximilian Richmond, who is revealed to have been the owner of the original source of the riches that made the seven Dragons wealthy.
- Wham Line: In The Stinger, Katrielle tells a riddle the Professor once gave her: "If you're not really my child, who exactly are you?"
- White-and-Grey Morality: Almost everyone in this game is good person, even culprit. They just have some reason to do their crime, or at least redeemable.
- You Could Have Used Your Powers for Good!: Katrielle says this about the Big Bad, Lord Adamas (in reality, Katrielle's assistant Ernest Greeves), who concocted a revenge plot against the Seven Dragons for supposedly causing his family to fall into financial ruin since she believes that there are better uses for his talents. She's well aware of what he can do since he was her assistant for the entire game.
