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 "Sheep King", the Sheep being 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.