Not long after Ugai Mori takes the reins of the Port Mafia, his partner, Osamu Dazai, has a horrific encounter with Chuya Nakahara, a boy known as the King of Sheep, who is capable of manipulating gravity. However, when strange legends about the mysterious being Arahabaki begin to spread throughout Yokohama, the duo must set aside their differences and uncover the truth. Before they gain the notorious reputation as the terrifying duo Twin Dark, Dazai and Chuya were just boys. Was their first encounter a harbinger of hope or a bad omen for what is to come...?