Ryuunosuke Akutagawa lives in the slums, using his ability to beat up anyone who dares lay a finger on his sister, Gin, or his friends. But his life takes a turn for the worse when armed criminals come after his gang, killing them in the process. In an attempt to avenge his friends, Akutagawa finds himself on the verge of death. A mysterious man dressed in black greets him, saying that he will take care of his sister until Akutagawa is ready to challenge him again.

More than four years have passed, and there is still no sign of the man in black or Gin. Even so, Akutagawa refuses to give up on his search. When Sakunosuke Oda takes pity on his living conditions and invites him to become a member of the Armed Detective Agency, an opportunity arises for Akutagawa to finally get a lead on Gin's whereabouts.

Unbeknownst to him, however, Gin is guarded by none other than Port Mafia's wicked leader, Osamu Dazai, and his successor, Atsushi Nakajima—the infamous "White Reaper." To save his sister, Akutagawa must overcome his weakness and sacrifice it all.