Fifteen-year-old Chuuya Nakahara commands a ragtag defense group called Sheep in Yokohama's lawless Suribachi district, using his gravity-manipulation ability to hold the peace. When rumors spread that the Port Mafia's dead former boss has been seen alive again, current leader Ougai Mori sends the equally young Osamu Dazai to investigate, and Dazai's meddling gets Chuuya captured and coerced into helping in exchange for Sheep's safety. Despite loathing each other on sight, the two form an uneasy partnership as a destructive force called Arahabaki stirs beneath the district. This prequel is set years before Chuuya and Dazai's later rivalry as mafia and agency.