Atsushi Nakajima/Synopsis

Armed Detective Agency vs. Port Mafia Arc
Aimless and starving, Atsushi arrives in Yokohama two weeks after being kicked out of his orphanage. Although plagued by the orphanage staff's cruel words, he refuses to submit and die. Initially reluctant, he decides to steal in order to survive. He intends on waiting for the next person to pass him by the Tsurumi River's bank, only to come across a young man drowning in the river and ends up saving him.

Quickly coming to, the man is displeased with being saved. Atsushi responds with great confusion, and the man clarifies he was trying to commit suicide by drowning. However, at the same time, he wanted his suicide to go without disturbance. As Atsushi gets entangled in his attempt, the man intends on doing something in return. He hears Atsushi's stomach growl and relates to the feeling, claiming he has not eaten in a few days himself. However, the river swept up his wallet, leaving him without money.

Soon after, Atsushi meets the man's colleague, Doppo Kunikida. Eventually, the two strangers end up treating Atsushi to chazuke in exchange for saving the suicidal man's life. The man finally properly introduces himself as Osamu Dazai. During the meal, Atsushi learns Dazai and Kunikida are detectives of the gifted Armed Detective Agency.

Atsushi learns their current investigation involves a giant white tiger that Atsushi remembers as one that has been following and taunting him lately after being kicked out. After forceful persuasion via Kunikida, he reveals the tiger was the cause of severe damage to the orphanage he was kicked out of. Now, it appears the tiger followed him all the way to Yokohama.

Using reward money as a bribe, Dazai pulls Atsushi into a plan as bait to capture the tiger once and for all. However, during their wait, an agitated Atsushi is at a loss for words when met with Dazai's complete nonchalance towards the tiger's looming threat. As moonlight pours into the abandoned building, Atsushi himself transforms into the aggressive white tiger. However, Dazai uses his ability to nullify the tiger, returning Atsushi to normal.

He wakes up in a dormitory and is called by Dazai, the one who set up his lodgings. Dazai claims he is about to die, and Atsushi quickly moves to check the scene, only to be met by yet another failed suicide attempt of Dazai's. After helping him, Atsushi meets with Kunikida as well, who informs them of a mad bomber at the Agency.

At the site, a young bomber holds one of the part-time clerks hostage and demands to see the Agency's president. Dazai and Kunikida plays rock-paper-scissors to determine who tries to diffuse the situation. After losing, Kunikida approaches the man, but he recognizes Kunikida and his ability, demanding he does as he says or else the bomb detonates. At a stalemate, Dazai makes Atsushi approach the young man to take care of the situation himself.

Terrified and nervous, Atsushi ends up going on a personal rant. He understands both he and the young man have nowhere to turn to. He also urges him to try finding work or another purpose instead of harming others because of his own misfortune. His intensity disarms the young man's defenses and Kunikida subdues him with his ability, The Matchless Poet.

Relieved, Atsushi is caught off-guard by an unseen figure pushing him, making him fall on the detonation device. Panicking, Atsushi uses his own body to lessen the bomb's impact, much to Dazai's shock. The bomb, however, does not go off. The bomber stands next to Kunikida and Dazai who both comment on Atsushi's impulsiveness.

The clerk, Naomi Tanizaki, hugs the bomber who turns out to be her older brother, Jun'ichirō Tanizaki. Confused, Atsushi questions the situation, to which Dazai reveals it to be his entrance exam for the Armed Detective Agency. The Agency's director, Yukichi Fukuzawa, arrives and confirms Atsushi passing the entrance exam, though he leaves the final decision to Dazai. Atsushi notices Naomi with the prop arm that pushed him onto the fake bomb, bewildering him with the elaborate setup of the exam. Dazai retorts that, if he declines, Atsushi has to leave the dorm with nowhere else to go, giving Atsushi no other choice but to accept the job.

At Café Uzumaki, Tanizaki profusely apologizes to Atsushi for causing him so much stress. Kunikida advises Atsushi not to ruin the Agency's name, while the Tanizaki siblings introduce themselves. Atsushi asks them what they did before joining the Agency, but Dazai suggests he guess their backgrounds.

Atsushi correctly assumes Tanizaki and Naomi to be students, while he guesses Kunikida to be a former government worker. Dazai reveals that Kunikida is a former math teacher, afterwards asking Atsushi to guess his former job. Kunikida remarks that no one knows his past job, having a monetary reward for the one who guesses it right. Amazed by the large sum, Atsushi eagerly guesses, but he ultimately fails.

Tanizaki receives a call from the Agency with a request from client. Her issue is a group of dangerous men hanging around her company's building. After the explanation, Kunikida assigns the investigation as Atsushi's first mission and orders Tanizaki to accompany him; Naomi joins them as well.

Before Atsushi and the company depart, Kunikida warns him of Ryunosuke Akutagawa and the Port Mafia. Kunikida informs him that Akutagawaa has an ability solely for murder, and the Agency cannot handle him. Thus, combat against him is strictly discouraged, admitting himself to be of no match for Akutagawa.

Led by their client Higuchi, Atsushi, Tanizaki, and Naomi later find themselves in a secluded alley. She reveals herself as a Port Mafia member, specifically Akutagawa's subordinate. She opens fire on the group, and the sight of Naomi's severe injuries brings Atsushi to his knees and at a loss for the situation.

His fear worsens after Akutagawa himself arrives, having attacked Tanizaki from behind. Akutagawa elaborates the Port Mafia's target is Atsushi himself, intending to claim an unknown third party's bounty on him alive, much to Atsushi's confusion. Akutagawa tells Atsushi that it is his fault for causing trouble to his comrades, bringing in flashbacks of his orphanage headmaster's harsh words condemning him for living.

After witnessing Akutagawa's ability Rashomon, Atsushi stumbles on the ground. Tanizaki, still conscious, urges him to run away. Instead, Atsushi's impulses drive him to narrowly avoid another attack, where he grabs Higuchi's gun to try and take Akutagawa down. Rashomon devours the bullets, making it impossible for Atsushi to defeat Akutagawa in this situation.

Rashomon severs Atsushi's right leg, causing him to relive another flashback of the headmaster's abuse. Here, the headmaster strikes and admonishes Atsushi for crying in pain, telling him no orphan abandoned by his own family has the right to cry. Although Atsushi knows that he has been abandoned and mistreated his whole life, he acknowledges that he has never given up regardless of the fact.

The tiger takes over and regenerates Atsushi's leg, impressing Akutagawa. When it appears Akutagawa kills him, it turns out to be one of Tanizaki's illusions. The real Atsushi attempts to ambush Akutagawa from behind, but Dazai arrives and nullifies both their abilities, rendering Atsushi unconscious.

Atsushi later awakens in the Agency's medical ward, with Kunikida seated beside him. After he remembers what happened, he immediately asks about Tanizaki and Naomi, Kunikida informs him that they are alright. When the two begin discussing the bounty on Atsushi, Kunikida explains that they will not waver and fight the mafia. However, Atsushi notices Kunikida's seeming anxiety and starts to feel guilty about putting the Agency in danger. After asking Atsushi to think how he can help, Kunikida leaves the room.

After hearing a distant explosion, Atsushi goes down the street and learns that it was done by the mafia's combat division, Black Lizard. He worries about what would happen if they attacked the Agency, so he calls Higuchi using her business card and tells her that he is quitting the Agency, urging them to find him alone.

He later bumps into Kunikida when he is leaving the office with his belongings, but he rushes back when he hears gunshots coming from the direction of the Agency, only to find that the detectives had utterly trounced the mafia's Black Lizard. As Kunikida complains how the raid being the "worst-case scenario" requires them to apologize to the neighbors, Atsushi begins to cry in relief realizing the Agency's true strength and that he has found a place to belong.

While Atsushi helps clean the office with the rest of the agency, he notices that Ranpo receives a ton of praise despite not helping and insulting the others. He is then asked to aid Ranpo on his next job, who states he does not need an assistant but someone to help him using the train.

The two of them later arrive at the murder scene and meet with officers Minoura and Sugimoto, the former stating they do not want their help. The victim, Minoura's subordinate Yamagiwa, was found in the river early in the morning with three bullet holes and no other evidence. Another body is caught in the police search net, but it turns out to be Dazai. Atsushi asks if Dazai was trying to commit suicide again, but he explains that he was not.

Shortly after Ranpo teases Sugimoto trying to get him to solve the case in 60 seconds, Atsushi thinks to himself that he probably looks that flustered most the time. He is also unimpressed by Ranpo's confident motto compared to his own. After Ranpo determines that Sugimoto is, in fact, the murderer using his ability Super Deduction, Dazai pushes Atsushi towards the armed officer, causing him to miss his shot. Atsushi then manages to disarm him and pin Sugimoto to the ground.

At the police station, Atsushi witnesses Sugimoto confess to the murder, explaining that he was working for a Diet member who ordered him to stop Yamagiwa from exposing his corruption. Ranpo later closes the case after Sugimoto reveals the location of the corruption evidence. Leaving the station, Dazai explains to Atsushi that Ranpo's Super Deduction is not an ability and merely solves crimes with his own intellect. Atsushi then realizes why Ranpo is never admonished despite his attitude.

In the morning, Atsushi is worried that Dazai is missing, but Kunikida, Ranpo, and Kenji Miyazawa think that he might be elsewhere like usual. To help appease his worry, Tanizaki volunteers to help him look for Dazai. Atsushi is glad to see him safe, but Tanizaki nervously relives Agency doctor Akiko Yosano's "treatment" and advises Atsushi never to get injured. Ranpo urges him to have a keen sense for danger, such as what will happen in ten seconds. Yosano arrives and sees no one else with Atsushi (which he thinks to be "danger"), so she asks him to accompany her for shopping.

After their spree, they ride the train back to the Agency, where they get embroiled in a bombing situation perpetrated by the mafia. They split up to find and diffuse the bombs, upon which he encounters a girl in kimono whom he encountered earlier. As he tells her about the situation, Atsushi is attacked by a katana-wielding phantom, which is the girl's ability. Launching a barrage of attacks on him, the girl introduces herself as Kyōka Izumi, a Port Mafia assassin whose ability receives instructions through her phone. Atsushi thinks he stands no chance against his opponent, but he decides to stand back up in order to save the passengers, believing that he can find his reason to live if he does so.

He partially transforms his arm into the tiger's limb and defends himself from Demon Snow. After dodging successive attacks, Atsushi demands the girl to deactivate her ability and tell him where the bomb is. She reveals the bomb strapped beneath her kimono, to which Atsushi berates her emotionlessness and apparent coldness. Through the PA system, Yosano tells Atsushi that the bomb has a detonator that can disarm the bomb. He asks Kyōka for it and presses it, but the bomb continues to count down.

The voice on the phone orders her to show the people the might of the Port Mafia by letting the bomb go off, so Kyōka pushes herself away from Atsushi and jumps off, saying her true emotions not wanting to kill anymore. Atsushi realizes her genuine feelings and, tapping into his ability, jumps off the train as well and rips the bomb off the girl, protecting her from its blast.

Kyōka is brought and treated to the Agency infirmary, while Atsushi is reprimanded by Kunikida for bringing in a wanted serial killer. As they ask her for information about the mafia, Kyōka asks them to treat her to tofu if they want her to provide them intel. As Atsushi ends up having his money thinned out due to Kyōka's appetite, she explains that she was taken in by the mafia after getting orphaned and used her ability for assassination missions. Moreover, they find out that the voice giving instructions through her phone is Akutagawa.

Outside, Kunikida asks Atsushi to hand her to the military police despite Atsushi's disapproval, knowing that she shall get capital punishment for killing 35 people. Kunikida also advises him not to let someone else beyond saving into his boat lest they drown together, but Atsushi wishes to give her another chance at life, just like how Dazai saved and took him in.

Atsushi accompanies Kyōka out to the streets and inadvertently tells her that they shall go on a date. He treats her to crêpe, gets her some gifts, and takes her throughout Yokohama. As Atsushi's wallet thins out, they soon arrive at their last stop – a police station – and she thanks him for his company. Atsushi worries that she will get executed if she proceeds, upon which he is suddenly impaled from behind by Akutagawa, who reveals to have tracked Kyōka's phone to kidnap the weretiger.

Atsushi wakes up inside a trafficking ship and gets confronted by Akutagawa, wondering where his hatred stems from. As the mafioso beats him up, Kyōka soon intervenes, but Akutagawa easily overwhelms her. Atsushi spots Kunikida below to rescue him, while Kyōka sets off a bomb nearby to sink the ship. Kunikida asks Atsushi to jump and leave the girl behind, but Atsushi firmly decides to give her a second chance and turns back to save her.

Before Akutagawa can kill Kyōka, Atsushi arrives to save her and urges the mafioso that they settle their matter at once. Akutagawa gets the upper hand with his strong offensive force, but Atsushi later retaliates with his ability. Akutagawa persistently attacks him and intermittently expresses his animosity towards him. Despite Rashomon's continuous attacks, Atsushi's regeneration enables him to survive, later berating Akutagawa for leading Kyōka down a dark path rather than directing her to another course. Atsushi soon defeats Akutagawa and plunges him towards the sea. As the ship sinks, Atsushi passes out, but Kyōka manages to get themselves aboard Kunikida's boat.

In the aftermath, Atsushi is glad that he and Kyōka survived, though he worries whether the Agency can forgive her 35 murders. To his surprise, he finds the Agency members playing with Kyōka and giving her treats, including Ranpo. Fukuzawa arrives to see the criminal in question, upon which Kyōka asks him to let her stay at the Agency. Atsushi also implores the President to help her out, which Fukuzawa accepts.

Minoura and his subordinate arrive at the office to ask for the Agency's help. He spots Kyōka and gets suspicious of her, causing Atsushi to improvise a story to get her out of apprehension. Fukuzawa simply tells the officer that she is his granddaughter, much to Atsushi's relief. Later, Atsushi and Kunikida learn about Kenji's rural town and their systems. Kunikida then tasks Atsushi to go with Kenji to investigate the recent explosion of a vehicle in town.

At the crime scene, Atsushi gets discouraged at the sight of the bizarre case, but Kenji tells him that they must first collect information. With Kenji's popularity throughout town, they soon set their eyes on a suspicious gang in question, from whom Kenji thinks they can extract the truth with their sincere questioning. Atsushi realizes how potentially effective his simple idea is. However, he gets terrified with the menacing gang, while Kenji straightforwardly asks for their involvement with the car explosion, which the gang denies. Kenji then accepts their answer and leaves with Atsushi.

Outside, Atsushi tries to point out their lies, but Kenji assures him that sincere communication will lead to understanding, which is a point that Atsushi acknowledges. On their way back, the gang corners them and “confesses” their involvement in the explosion after all. Kenji gladly thanks them, only for the gang to attack him from behind. As Atsushi panics, Kenji gets up unscathed and single-handedly defeats the entire gang with his monstrous strength. With the case closed, Atsushi calls Kunikida and expresses how too peculiar Kenji's methods are. Nonetheless, Kunikida explains to Atsushi what and how Kenji's ability works.

The Guild Arc
Atsushi wakes up and discovers that his roommate is Kyōka. At the office, Dazai advises him that it is his job to look after her, which Atsushi accepts. Dazai also informs them that the one who placed the bounty on Atsushi is a North American gifted organization known as the Guild. Tanizaki bursts in and tells them that the Guild has just arrived at their doorstep. Their leader, Francis Scott Key Fitzgerald, asks to purchase the Agency from Fukuzawa, an offer which the President rejects.

The following morning, the Agency are unsettled by the possible moves undertaken by the Guild, including how a mafia-affiliated building vanished overnight and how Kenji has turned missing after seeing them out the building. Kunikida tasks Atsushi and Tanizaki to search for Kenji, and Naomi accompanies them as well. As they look for Kenji along the streets, Atsushi and Tanizaki soon realize that Naomi has vanished.

They soon get transported to a different dimension along with other civilians by Lucy Maud Montgomery, an apprentice from the Guild. She shows the detectives their captured comrades and asks them to play with her ability, a gigantic doll-like creature named Anne. If they defeat her, Lucy promises to set their subordinates free. The other civilians flee from the room, leaving only Atsushi, Tanizaki, and a doctor behind in the dimension. As soon as Atsushi and Tanizaki accept her challenge, Anne instantly captures Tanizaki and imprisons him.

Atsushi dodges Anne's onslaughts with his ability until he gets his hand on the key to the prison room. However, it turns out to be a feint that even Lucy admits she does not know to use the key. Running out of ideas, Atsushi heads towards the exit to seek Dazai's help, but the doctor stops him and imparts some advice towards him on how to possibly defeat Lucy. As he earns back his resolve to save his comrades, Atsushi readies himself, but he gets caught by another Anne and dragged into the prison room.

Eventually, the doctor reveals to Lucy the truth: Tanizaki has projected an illusion of the door shutting close, while Atsushi actually resists getting dragged to the prison room with his tiger limbs. Atsushi also reveals having tied a ribbon on Lucy's waist earlier, then he pulls her towards him and demands she deactivate her ability. Otherwise, he will drag her together with him to the prison room. As Atsushi lets go, Lucy deactivates her ability and brings back the imprisoned people back to reality.

Atsushi tries to approach Lucy and apologizes to her, only for her to flee. Kyōka then arrives and embraces Atsushi out of worry. The doctor bids his farewell to Atsushi and imparts a final advice to him. As he and his young companion leave, Kyōka suddenly cowers out of fear, much to Atsushi's confusion and worry.

The next day, Atsushi hands Kyōka her first work for the Agency, which is to deliver an evidence to a court judge. As they arrive at the location, she is denied entrance, but she tries to resolve it using her own tactical strategy. Atsushi accompanies her to infilitrate the building, though she soon ends up stunning the judge himself, ending in a partial failure. Afterwards, Atsushi treats her to crêpe to cheer her up, later asking her why she still keeps her phone. She explains how it is special to her despite her wish never to activate her ability through it again. However, it suddenly rings, with a voice activating Demon Snow to attack Atsushi.

A woman, Kōyō Ozaki, arrives and tells Kyōka that she has arrived to bring her back to the mafia. Atsushi tells her that Kyōka shall never return to the mafia again, as she now belongs in the Agency. Atsushi activates his ability to attack the woman, only to be stabbed by another demon like Demon Snow, Golden Demon. She announces that she has to kill Atsushi and the rest of the Agency upon orders of mafia boss Ōgai Mori. Kyōka then agrees to come with her, though it turns out to be an act to catch Kōyō off-guard.

She snatches Kōyō's phone and activates Demon Snow to battle her opponent, as Kōyō keeps urging her to return to the mafia. She also reminds Kyōka how she is suited for killing, just like how Demon Snow killed her parents, much to Atsushi's surprise. In that moment, mafia reinforcements arrive to finish off Atsushi and retrieve Kyōka and Kōyō.

However, Kunikida and Kenji intervene and arrive to Atsushi's aid, inciting a war between the Agency and the mafia. Just then, two Guild members arrive alongside four more other members. Kōyō orders her men to open fire, though the Guild members easily overwhelm all the forces, including Atsushi and his subordinates.