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 pas