Bungo Stray Dogs: Dazai, Chūya, Age Fifteen

Bungo Stray Dogs: Dazai, Chūya, Fifteen Years Old (文豪ストレイドッグス 太宰、中也、十五歳,) is a light novel written by Kafka Asagiri, exclusively released during the official DEAD APPLE premiere on March 3, 2018. It narrates the story of Osamu Dazai's first encounter with Chūya Nakahara and how the latter joins the Port Mafia.

A booklet summarizing the light novel's events was included in DEAD APPLE's DVD and Blu-ray release on November 28, 2018. The light novel was also adapted into the anime, filling in Episode 26 to Episode 28.

Description
The ghost of the previous boss haunts the Port Mafia. At fifteen, Osamu Dazai, under the careful eye of Ōgai Mori, receives his first mission to investigate the matter. The investigation leads him to Chūya Nakahara, the infamous "King of the Sheep", a rival group of juveniles. With his own past unknown to him, and the lives of Sheep members on the line thanks to the Port Mafia, Chūya agrees to work alongside Dazai in order to unveil the truth behind the Port Mafia's former boss.

Summary
A mafioso aboard a passenger plane encounters an adolescent floating outside the plane, who assaults the man by manipulating gravity. The adolescent threatens the mafioso, who apologizes for attacking Sheep's territory after their own armory was likewise destroyed by the black flame of Arahabaki. As the adolescent is taken aback by this remark, the mafioso grabs his gun and shoots the King of the Sheep, Chūya Nakahara, but the latter easily overwhelms the man, after which he proclaims that he shall kill all of the Port Mafia.
 * Prologue

After secretly assassinating the previous mafia boss in front of Osamu Dazai, the new boss, Ōgai Mori, bids the 15-year-old prodigy not to die as his witness in his new position, though the boy retorts that he is quite bored with the world and wishes to die. Nonetheless, Mori asks Dazai, despite not being a formal member of the Port Mafia, to investigate recent rumors surrounding the reappearance of the supposedly dead mafia boss. Being handed a silver oracle, Dazai enlists Ryūrō Hirotsu to be his escort in their investigation.
 * Phase.01

Dazai and Hirotsu arrive in Suribachi City, where various people illegally settled after an unexplained explosion happened in the post-war. After interviewing witnesses about their target in the area, Chūya attacks Dazai and threatens him to spill what he knows about Arahabaki from their inquiries. Hirotsu retaliates against Chūya with his own ability, after which Dazai nullifies the boy's ability upon grabbing him. In the middle of their skirmish, the previous mafia boss emerges in a black flamed explosion, after which Dazai passes out due to his injuries.

Initially restrained by mafia sub-executive Randō, Chūya is brought before Mori, who invites him to join the Port Mafia, but Chūya declines. Besides not wanting to join an infamous group previously headed by a fiendish boss and presently led by a street doctor, he knows the rumor of the doctor killing his predecessor. Mori nonchalantly confesses to have done so, after which he decides to invite Chūya to a joint investigation instead. Upon learning that his subordinates are held hostage by mafiosi, Chūya reluctantly agrees to cooperate.
 * Phase.02

Mori shows a footage of the previous mafia boss infiltrating a secured mafia vault, with which Chūya tells them about its possible connection to Arahabaki, an uncontrollable deity that can be revived by people who killed others in life, as well as the cause of the explosion that created Suribachi City and its distinct topography. Dazai thinks that the video is created by an ability user who used the Arahabaki rumors to disguise the mafia boss's revival. Hence, Mori orders Dazai and Chūya to find the culprit together, which the two dubiously accept.

After bantering with each other en route to Randō's home, a survivor in the Arahabaki explosion years ago, Dazai and Chūya arrive just in time when Gelhart Security Service troops attack the place. Chūya fends them off, after which he and Dazai approach Randō in his room. They conjecture that GSS had planned on stirring discord in the mafia by attacking Randō under the guise of Arahabaki, only to fail. Randō then vividly narrates the Arahabaki explosion that he survived, after which Dazai states that he now knows who the real culprit is.

Leaving Randō's home, Dazai and Chūya head to the arcade to battle each other in a game as a deal: whoever loses must follow the other's orders like an obedient dog. In the end, they battle once more in a final game, when a Sheep trio approach Chūya to update him on their Arahabaki investigation. As they fetch him back, Dazai informs them that Chūya is currently under the Port Mafia's orders; Chūya also insists that he plans on catching the criminal in any case, making his silver-haired comrade think that Chūya has betrayed Sheep.
 * Phase.03

In an abyss where a darkness exists, the seal that confined it was suddenly broken by a figure. As if being summoned into the outside world, the darkness incarnated as a flame and incinerated everything as far as the eye could see out of anger, finally being born.
 * Phase.xx

Inside a shipyard building, Dazai and Randō make some advance preparations for Chūya's liberation from Sheep. As the two talk about the affairs in the underworld and Randō's time during the previous mafia boss's time, Dazai later names Randō as the criminal. To explain his allegation, Dazai has noticed an inconsistency in Randō's testimony that gave away his motive. Chūya then barges in also to name Randō as the culprit. Randō then implores Chūya to tell him where Arahabaki is, to which Chūya replies that he is Arahabaki.
 * Phase.04

Chūya accosts Randō as the one who broke the seal eight years ago, with which Randō isolates Chūya in his subspace from Dazai in another. Randō finally remembers his true name, Arthur Rimbaud; his former profession; and his true goal, which was to find and kill Chūya. Also hovering before Dazai is the previous mafia boss, who was humanized through Rimbaud's ability. Chūya's ability is rendered futile against his natural enemy ability, and Dazai is cornered by the mafia boss' scythe that launches physical attacks he cannot nullify.

Dazai then tells Chūya his plan to defeat Rimbaud that shall require their collaboration. The two crush the previous mafia boss, and, despite Rimbaud's overwhelming power, Chūya's and Dazai's collaboration has been taking effect all along, which eventually subdues Rimbaud fatally. Recalling his best friend and past partner Paul Verlaine's betrayal, Rimbaud harbors a change of heart and asks Chūya to live regardless what he is or whence he came from. Believing that all figures in the world are beautiful, Rimbaud succumbs to his death.

A month later, Chūya visits Rimbaud's gravestone by a cliff, where Shirase and the others locate him. Deeming it as their solution to Sheep's problem, Shirase stabs Chūya and reveals their alliance with GSS to kill Chūya in order to survive as an organization. Chūya musters his strength and escapes beneath the cliff with his ability, where he encounters the new mafioso Dazai and countless armed men. With no other choice and having understood Dazai's reason for arriving, Chūya accepts his deal and asks them not to kill the Sheep children.
 * Phase.05

In a corridor, Dazai walks with Mori and a young child named Kyūsaku, who has a highly dangerous ability that Mori asks Dazai to use his ability on when necessary. As Chūya is briefed by Kōyō Ozaki on an upcoming mafia meeting to garner him some experience, he encounters Dazai and his companions, and the two bicker once again. Mori earnestly looks at Chūya's black hat, the recruitment gift he gave him when he joined the mafia.
 * Epilogue

At that time, though Chūya initially thought lowly of the hat, he accepted it after learning it was Rimbaud's hat. Mori informed Chūya that he has information that Rimbaud left behind, including the truth on Arahabaki, albeit classified for executives only. Mori also advised that a leader is one who would deliberately dirty their hands for the sake of the organization, a quality Chūya admitted he lacked in leading Sheep. Chūya then vowed to serve him with utmost loyalty. Eventually, he rises as an executive and soon accesses Rimbaud's materials.

At present, the entire incident is shown to be narrated in a report by Ango Sakaguchi on the Arahabaki Incident. Later, a young man infiltrates the mafia headquarters' top floor and reads Rimbaud's Arahabaki documents. The man is fascinated upon reading the passage on Paul Verlaine's death, as he claims that he himself is still alive after all. After silently killing off the guards, he retreats into the darkness of the night and remarks that he must soon meet his younger brother, Chūya.