Dazai meets Chuuya Nakahara, the powerful leader of a juvenile group called the "Sheep". Despite their mutual dislike, they form a reluctant partnership to uncover the truth behind a mysterious explosion and a god-like entity known as Arahabaki.

After returning to the present day, the story shifts to a confrontation with a new threat: the "Rats in the House of the Dead," led by the mastermind Fyodor Dostoevsky.

After defeating the mastermind Arthur Rimbaud, Chuuya is betrayed by his own group and eventually joins the Port Mafia at Dazai's invitation. The Present: The Cannibalism Arc

The season opens with a three-episode flashback adapted from the light novel Dazai, Chuuya, Fifteen Years Old .