Some years after Ougai Mori took command of the Port Mafia, he orders the young Osamu Dazai to investigate an ominous rumor about something called "Arahabaki." Whatever it is, it seems to have some connection to Mori's late predecessor—and the new leader would very much like to keep the dead buried. Along the way, Dazai meets a gravity manipulator, Chuuya Nakahara, the leader of the Sheep gang. Together, the two will eventually be known as the terrifying duo Twin Dark, but for now, they must learn to put aside their differences if they want to uncover this mystery...