Osamu Dazai vs. Chūya Nakahara

Osamu Dazai vs. Chūya Nakahara is a battle fought between former Mafia partners, Armed Detective Agency member Osamu Dazai and Port Mafia executive Chūya Nakahara during the bounty hunt for Atsushi Nakajima.

It happened at the Mafia's infamous torture room that also serves as an underground prison where Dazai gets detained upon his "capture".

Prologue
While wandering at night, Dazai is caught by Mafia assassin Kyōka Izumi, who appears to activate Demon Snow and brings the former Mafia executive to their base. Amidst Atsushi's concern, Dazai's disappearance is overlooked by his fellow Agency members, who dismiss it as his typical personal business.

Dazai is shackled and confined in the Mafia's torture room, an act that does not apparently faze him. His former apprentice Ryunosuke Akutagawa keeps watch over him and threatens him and his comrades, but Dazai is unfazed and merely provokes Akutagawa by claiming that his new subordinate is much more capable than him. In a fit of rage, Akutagawa lashes out at him.

Eventually, Chūya visits Dazai and mocks his detained visage, much to the Dazai's grimace. Chūya asks his reason for allowing himself to be captured, to which Dazai tells him that he got caught and is now awaiting execution. Chūya insists Dazai to divulge his true motives, knowing full well that Dazai, his former Mafia partner, is too cunning to merely be captured quite easily.

Dazai dodges his question; in turn, he asks Chūya why he has gone to visit him. Chūya asserts that he has come to spite him in an attempt to return the same spite that Dazai inflicted on others in the past, as well as to prevent Dazai's plan whatever it may be. Hence, Chūya releases Dazai and demands to duel him. As Dazai reveals having picked the lock to his chains long ago, Chūya notices that he could have escaped whenever he wanted, further piquing his interest.

Battle
Chūya charges at Dazai, who dodges his moves. Dazai grabs his arm and punches Chūya, but the latter is unhurt by his weak punch and effortlessly kicks him off, sending him flying against a wall. Chūya teases that his punch cannot even compare to a massage, befitting someone whose combat skills were below average in the Mafia.

Chūya claims how troublesome his nullifying ability is, though it is quite powerless against physical attacks, which does not even require him to activate his ability. He bids Dazai to stand back up, with which the latter gets up, acknowledging Chūya's strength as the Mafia's best martial artist that almost ripped off his blocking arm. Chūya then realizes that Dazai has been reading his moves, having almost efficiently blocked his previous kick.

Hence, Chūya swiftly rushes towards Dazai and punches him on the face, with Dazai unable to anticipate it. He launches more punches at Dazai and eventually pins him back on the wall. Chūya holds Dazai at knifepoint, asking him once more why he let himself get captured. Initially silent, Dazai soon tells him it was for Atsushi, professing that he wishes to know the one behind his bounty.

The Mafia executive chides Dazai for being quite different from his past self as the youngest Mafia executive in history, even putting himself in danger for a comrade as he claims. Dazai chuckles and warns Chūya that a five-executive meeting will commence the following day after he delivered an incriminating letter stating that all the Mafia's secrets shall be spilled to authorities if harm or death comes to Dazai.

Aftermath
Chūya finds himself with his back against the wall due to the circumstances having played out exactly according to Dazai's plans. With Chūya having antagonized Dazai before the five-executive meeting can even commence to decide their next action, he can be penalized for treason for harming Dazai, which (as stipulated in the letter) shall trigger the Mafia's secrets to get divulged to authorities. Moreover, despite Chūya's desire to kill Dazai, he finds it useless because of Dazai's own wish for death as well.

Chūya soon realizes that Dazai must have also expected his arrival to force him to his hardest dilemma, though Dazai claims that he merely wishes to throw a surprise for their reunion. He then yields and tells Dazai where he can find the details of the responsible party on the bounty. As he takes his leave, Dazai reminds something he has forgotten. Like how Dazai earlier predicted that Chūya shall leave the room empty-handed and walk out like a girl with his feet turned in, Chūya begrudgingly acts as such and warns Dazai that he shall have no second chance next time.

Manga and Anime Differences

 * In the manga, Dazai laughs after Chūya tells him that there will be no next time, while he does not have any reaction in the anime. This angers Chūya, who demands Dazai to laugh.