The story is set at the end of the 19th century in a world where supernatural beings exist. Aya Rindo, a disembodied head carried by her loyal maid Shizuku Hasei, requests the half-demon demon slayer Tsugaru Shinuchi to help her find her missing body in Europe as she believes that her assailant is there. As they travel, they solve supernatural cases and meet various figures along the way while looking for Aya's body.
Tropes:
- Actually, That's My Assistant: Because Aya is hidden from the general public, the reputation of The Cage User detective usually goes to Tsugaru and Shizuku, even though their main job is to carry her around. Amongst detective circles, Aya is well-known on her own.
- Adaptational Jerkass: Compared to the novel and manga version of Aya Rindo, who is more innocent and cheerful in her dialogue, the anime version wears a near constant Death Glare and often snarks at Tsugaru's expense.
- Adaptational Muscles: Compared to his appearance in the manga, which made him look younger and more slim, Tsugaru in the anime looks older and more muscular.
- Arbitrary Skepticism: Lord Goddard and his sons, who are vampires, are shocked to see that Aya is a living severed head. Raoul, the youngest, wonders how she's still alive.
- Arch-Enemy:
- Just like in the original canon, Professor Moriarty is Holmes' archnemesis. He is also considered to be Aya's archnemesis as well as given that he remembers her.
- As revealed in the final episode, Carmilla becomes this for Shizuku given their previous duel. And when Carmilla escapes again, Shizuku swears she'll kill her with her gun.
- Badass Normal:
- Humans don't stand a chance against monsters, so beside Royce, only Shizuku has shown enough fighting capability. Arsene manages to beat Tsugaru, but before it's revealed Tsugaru was Willfully Weak and keeping his strength as an ace.
- Without any supernatural abilities or magic, Sherlock manages to defeat Crowley with the use of his Baritsu.
- Bayonet Ya: Shizuku's rifle has a bayonet attached to it, which she liberally employs in her scrap with Tsugaru.
- Big Bad: "The elderly foreigner with the M cane", Professor Moriarty, has ruined both Aya's and Tsugaru's lives and continues to create mystery cases in Europe.
- Blonde, Brunette, Redhead: At Wolfinhöhle, there's three female werewolves with these hair colors; the blonde Nora, the brunette Kaya and the redhead Vera.
- Casting Gag: This isn't the first time Shin-ichiro Miki voiced Sherlock Holmes in the show. He also dubbed Sherlock in Elementary.
- Commonality Connection: On their first meeting, Aya and Tsugaru realize that they have the same desire of wanting to die. They also discover that the assailant who beheaded Aya is also the one who injected demon blood into Tsugaru because they both remembered that his cane is engraved with the letter, "M", who is suspected to be James Moriarty.
- Death Seeker:
- Considering that she's lived for 947 years and has recently been reduced to a severed head, Aya seeks out Tsugaru at the start of the series to kill her. Tsugaru refuses, and instead suggests they go after the person who beheaded her, as he was also responsible for Tsugaru becoming a half-oni.
- Aya deduces that Tsugaru wants to die which the latter doesn't deny given that he keeps rushing to his death during gladiator fights with demons in his troupe. And if he eventually succumbs to becoming a full demon someday and perishes, he plans on having fun doing so by slaughtering the vile audience members of his shows when he finally turns.
- This is implied in the final episode on Louise's reason for helping Nora in her plot. She grows tired of being isolated in the village which doesn't help that her parents planned to abandon her because of her disability. She also feels guilty of exposing Nora which led to Rosa's death and believes Nora should punish her for what she did.
- Defensive Feint Trap: Tsugaru's first fight against Lupin is nothing more than a feint to trick Lupin.
- The Exotic Detective: Aya is a detective who is a living head and has to rely on Shizuku and Tsugaru to carry her in a cage so she can look at the evidence.
- Fair-Play Whodunnit: The cases that Aya handles give a lot of clues which encourage the audience to solve as well. It helps that the show and the manga divides the cases into episodes and chapters to give time for the audience to analyze the clues and theorize.
- Fake Wizardry: Aleister Crowley presents himself as a warlock, but he's actually more of a combination of Stage Magician and alchemist.
- Famed In-Story: During their travels in Europe, Aya and Tsugaru are already well-known for being detectives, dubbed as "the Cage User".
- Fantastic Racism: Due to the supernatural existing in the world, both humans and monsters looked down on each other. As Tsugaru summarized, the two species cannot coexist.
- Fiction 500: Phileas Fogg is far, far wealthier then in the original novel he comes from, which enables him to set a ridiculously well-fortified security system.
- Final Solution:
- In Japan nearly all non-humans have been purged by the authorities and have to live in The Freakshow.
- Even out of Japan, the Royce organization are utterly dedicated to the same end goal, all while operating under the guise of a security agency.
- Flawed Prototype: Tsugaru was this in the experiments to create half-onis as he was made to have more oni blood than human, which results in him facing the risk of becoming a complete oni and going berserk.
- The Freakshow: Tsugaru used to live in an illegal circus where he had to fight monsters for the rich audience. Other humanoid monsters were kept for other kinds of entertainment.
- Glass Cannon: What Aya describes onis as, because while they lack many other supernatural entities' durability and can easily be done in by a gunshot or explosion, their overwhelming power can deal fatal damage to all kinds of creatures regardless, and even harm immortal Fushis like her.
- Gory Discretion Shot: The anime didn't show much of Jack the Ripper slaughtering the police force. The camera immediately cuts away from the scene whenever he makes a kill.
- Historical Badass Upgrade: Jack the Ripper becomes deadlier as an Oni hybrid just like Tsugaru. This is all thanks to Moriarty's research on Aya's body which allows him to perfectly combine with the Oni cells and take in other monsters' abilities. What's even worse is that the Immortal cells can neutralize the monsters' weaknesses, making Jack more dangerous.
- Historical Domain Character: Aleister Crowley and Jack the Ripper appear in the story as part of a criminal organization, called "Banquet", headed by Moriarty.
- The Igor: Tsugaru is a strange, rare heroic example, serving mistress Aya with great enthusiasm, with the caveat that Aya is a detective, not a Mad Scientist (but both professions are still heavily associated with geniuses). His usually hunched posture and penchant for other bizarre Igor-esque mannerisms help make him look even closer to the archetype. In return, Aya provides him with the means of extending his lifespan, while also granting him a more meaningful (or at least interesting) life.
- Immortality Begins at Twenty: Aya stopped aging after gaining her powers as a Fushi at the age of 14, resulting in her being nearly a thousand years old yet only looking like a teenager.
- Immortal Breaker: As a part-oni with more oni blood than human, Tsugaru has the ability to ignore the immortality of beings he attacks. It's for this reason Aya seeks him out at the start of the series, as she wants him to use his ability to kill her because another oni stole her body.
- Intrepid Reporter: Annie Kerber is a young reporter from the French newspaper, Epoch, who writes about odd stories including the Cage User's cases. She happens to be acquainted with the trio.
- Intimate Healing:
- The easiest way for Tsugaru to consume Aya's body to stave off his eventual transformation into an oni is by ingesting her bodily fluids, and Aya accordingly gives him her saliva via kissing him.
- After Shizuku falls into a rives and drifts into Wolfinhöhle, two female werewolves keep her body warm by sleeping nude with her.
- Ironic Nickname: Tsugaru's moniker as the "Oni Slayer" is this as a result of being part of a group that hunted down monsters during Meiji Japan's purge of the supernatural prior to becoming a half-oni, which was aptly known as the "Oni Slayers".
- Irony: Tsugaru is asked by the villagers to get rid of a stray cat, which he refuses because it's not a monster. His next opponent in the arena is a nekomata, something he comments on.
- Jerkass to One: Tsugaru's more laid-back attitude and willingness to joke around with Aya doesn't win him any points with Shizuka, who otherwise treats everyone else with courtesy, even if she does remain quiet throughout.
- Kill It with Fire: Aleister can set people on fire by fingersnaping in their direction. In truth he throws a needle with a special liquid.
- Last of Her Kind: Aya is apparently the last existing Fushi.
- Let Off by the Detective: In the first season's final episode, Aya lets Nora/Jutte escape; while Nora killed three innocent human girls and deserved to be punished, she did so to free her sisters from a Fate Worse than Death, as her clan had crossed the
Moral Event Horizon and raped the girls with the whole village out of a twisted obsession with eugenics. As seen from the massacre caused by outside interference, Nora is one of the few people who can continue the fight against her clan without exterminating them all. - Lesbian Vampire: While she is shown flirting with both male and female targets, Carmilla prefers blood from females.
- Living Aphrodisiac: Carmilla's blood is laced with an aphrodisiac which can weaken her target once they touch her body and blood. Shizuka finds this out the hard way.
- Living Legend: Even though the plot takes place several decades years after Fogg's famous journey around the world with his butler Passpartout, Fogg is still one of the most famous living people in the present.
- Long-Lived: Aya has been permanently "14 years and 3 months" for 947 years and has the appropriate intellect for someone living for a millennia.
- Losing Your Head: This is what happened to Aya prior to the story. She was beheaded by an unknown assailant and her body was taken away. However, her status as an immortal Fushi, along with her attacker being a half-demon and unable to kill her off, resulted in her surviving as a severed head. Her head has to be carried by her maid, Shizuka, who placed it inside a birdcage.
- Male Gaze: The anime enjoys focusing attention on Carmilla's Impossible Hourglass Figure when she's in the scene.
- Mass "Oh, Crap!": After the Banquet celebrates their victory for stealing the diamond, Jack reaches into his pocket to take the diamond out. But much to the shock of everyone, his pocket has a hole, meaning Tsugaru cut his pocket when Jack wasn't looking and stole the diamond right under his nose.
- Massive Multiplayer Crossover: The setting combines a lot of stories set in Victorian London's time period, from Sherlock Holmes, to Around the World in Eighty Days, to The Phantom of the Opera.
- Master of Disguise: Obviously, Lupin is known to be good at disguise and even dresses up as Sherlock until Mycroft Holmes arrives and points out the flaws of his disguise. He also dresses up as his rival, Inspector Ganimard, so that he can sneak inside Fogg's mansion. However, Sherlock manages to figure out his true identity.
- Mauve Shirt:
- In Episode 8,one of the Royce agents, Fatima Doubledarts, gets brutally killed by Jack the Ripper.
- In the final episode, the two Royce agents, Kyle and Alice, are both neutralized by Tsugaru, Victor, and Crowley.
- Mêlée à Trois: Episode 7 has Lupin VS Tsugaru VS Reynold Stinghart fighting over the diamond. It would have been just Lupin VS Tsugaru if Stinghart hadn't intervened to get rid of both of them.
- Monster Mash: Moriarty's group includes other literary villains like Carmilla, Jack the Ripper and Victor Frankenstein.
- Naginatas Are Feminine: While it's not an actual naginata, the bayonet on Shizuka's musket nonetheless allows her to use it like one in close quarters.
- Nice Job Breaking It, Hero:
- Sherlock manages to capture and trick Lupin who disguised himself as Ganimard and hides the diamond from his reach. Unfortunately, due to his arrogance, Sherlock tells everyone where the diamond is (Watson's jacket) while Lupin is still in the room, uncuffed. Because of that, Lupin successfully steals the diamond and flees from the scene until Aya and Tsugaru ambush him and Phantom in the greenhouse with Aya hiding inside the safe.
- Aya and Tsuguru find a secret underground passage that connects to the human village and the werewolf village. Unfortunately, the two Royce agents, Kyle and Alice, also find the opening of the passage which Aya and Tsugaru used to go back into the werewolf village and lead the villagers into the werewolf village where they massacre the werewolves. Later on, it's subverted when it turns out Jutte is the one who discovered the passage in the first place and left it open.
- Ninja Maid: Shizuku is Aya's bodyguard and has combat skills to match the Oni Slayer Tsugaru.
- O.O.C. Is Serious Business: After Tsuguru mistook a parrot bird cage for Aya's cage, Aya ends up being abducted by the red-headed twin criminals. Despite her calm and snarky attitude, she goes into a panic and screams Tsuguru's name as the twins take her away.
- Occult Detective: Given the series' story where supernatural beings exist, the trio solves cases related to the supernatural. The first case presented in the story is the murder of a vampire.
- Parallel Conflict Sequence:
- At the end of Episode 7, Aya and her allies are facing off against Moriarty's group starting with Aya, Sherlock, and Watson who face Crowley while Shizuku and Lestrade confront Carmilla. Meanwhile, the Royce agents are busy dealing with Lupin and Phantom with Tsugaru caught in the middle.
- The end of episode 12 shows Kyle facing Victor while Alice bumps into Crowley who just finished burning up a werewolf. Meanwhile, Shizuku finds Kaya being drugged by Carmilla, who is waiting for her.
- Playful Cat Smile: Tsugaru has a habit of making these whenever he is goofing around or is trying not to frighten bystanders.
- Public Domain Character: Some of the main cast are known literary figures who are in public domain such as Sherlock Holmes, Arsène Lupin, The Phantom of the Opera, Carmilla, Frankenstein's Monster and the two main characters of Around the World in 80 Days. Dracula is also mentioned and Tsugaru is the one who killed him.
- The Quiet One: Shizuku rarely talks unless spoken to. Many people who are unfamiliar with the trio, including Tsugaru during his first encounter with her and Aya, often mistake Aya's voice for Shizuku doing ventriloquism.
- Red Herring: Alma, the painter who lives at the end of the human village, is suspected to be the murderer of the three girls and the villagers saw her transformed into a wolf, confirming their suspicions. However, the last two episodes reveal she's a scapegoat used by Nora in an attempt to trick the villagers, the Royce agents, and the Cage detectives. Eventually, Aya and Tsuguru find Alma dead in the underground passage.
- Red Oni, Blue Oni: Tsugaru is the Blue oni to Jack's Red oni. Their character designs are anything but subtle about it, both of them having lines across thier body, with Tsugaru having a horizontal line stretching across the left side of his face, while Jack has one on the right side of his face. Of course, Tsugaru's lines are sapphire blue, while Jack's are crimson.
- Seen It All:
- In the anime, Sherlock is unfazed about Aya being a living severed head which is in contrast to the manga where he's shocked to see her.
- Lupin mistakes Annie for Aya Rindo until he actually meets her face to face. He's not surprised about her being a disembodied head.
- Sequel Hook: The show ends with Aya letting Nora go as she, Tsugaru and Shizuku plan to go back to London where her body is located in Moriarty's home base.
- Sherlock Scan: Despite being a disembodied head, Aya has good observational skills which she can use to make deductions. For example, she was able to figure out what Tsugaru wanted in moments during their first meeting by just observing him. Her skills are even compared to Sherlock Holmes, though Aya insists that Holmes is way better than her.
- Single-Stroke Battle: Carmilla can slash police officers into pieces with a sword without visible movements.
- Smarter Than You Look: While Tsugaru is far more irreverent and carefree compared to Aya and Shizuku, he's no slouch when it comes to investigations as Aya's assistant and can very much keep pace with her observations. Even when they first met, he's able to quickly piece together that it was no coincidence that they wound up meeting, and is the one to make the counteroffer of them teaming up to find whoever was responsible for their current states as a half-oni and severed head.
- He later demonstrates this again by effortlessly stealing back the Penultimate Night from Jack the Ripper in what looks like a failed attack, slicing open the pocket it was in and hiding it in his sleeve, before discarding his jacket for the real fight, all while making himself look like nothing more than a superpowered circus performer.
- Smug Snake: After stealing the diamond and having Phantom steal the safe as well, Lupin believes that he won until Aya, who is inside the safe, opens the lid from the inside, hitting Lupin in the face.
- Spared by the Adaptation: In this story, Moriarty has survived his showdown with Sherlock in Reichenbach Falls.
- Spit-Trail Kiss: Aya leaves behind a strand of saliva when she's done exchanging saliva mouth to mouth with Tsugaru.
- Stepford Smiler: Tsugaru is loony and prone to comedic theatrics, but if not for chasing Moriarty he'd make himself killed due to having nothing else left.
- Summation Gathering:
- In the fourth episode, Aya has the Godard family and the servants gathered so they can listen to her summation regarding Hannah's murder.
- In the final episode, Aya stops the fight between the human villagers and the werewolves and reveals the true culprit behind the murders.
- Super Breeding Program: The werewolf tribe seeks to breed the Kindsfurher, an apex version of werewolves. So far, the successful branches have overcome their immunity to silver. It turns out that Jutte is their first success - her reaction was to try to destroy both villages after everything they put her and her sisters through.
- Town Girls: The three female werewolves that befriend Shizuku are The Stoic Nora, the feminine Kaya and the tomboyish Vera.
- Truer to the Text: One of the few depictions of Professor Moriarty in modern fiction as an old man, just like he was in the original novels.
- Undying Loyalty: Shizuku is very loyal to her mistress. As she explains to Tsugaru, if her mistress wants to die, she'll find a way to achieve it.
- Victorian London: The story is set at the end of 19th-century London.
- Villain: Exit, Stage Left: After Jack shoots out a flare in the sky signaling that he has the diamond, Moriarty and the rest of the Banquet members retreat not before Moriarty gloats to Holmes, Watson, and Aya to try and stop him.
- Wham Episode: Episode 8 shows the confrontation between Aya and Moriarty who reveals his reason for possessing Aya's body and how he uses it to empower Jack the Ripper and create the perfect chimera along with other monsters.
