Living in the slums, Ryuunosuke Akutagawa relies on his fighting skills to protect his sister and friends. But when his gang is killed by armed criminals, Akutagawa seeks revenge and is left near death. A mysterious man promises to care for his sister until Akutagawa is ready to confront him again.
Years later, Akutagawa still searches for the man in black and his sister Gin. He is offered a chance to join the Armed Detective Agency by Sakunosuke Oda, which may lead him to Gin. However, unbeknownst to him, Gin is under the protection of Port Mafia's Osamu Dazai and Atsushi Nakajima, the "White Reaper." To rescue his sister, Akutagawa must confront his weaknesses and make a sacrifice.
