In a parallel Yokohama, Ryunosuke Akutagawa carries the Armed Detective Agency's badge while Atsushi Nakajima grows up inside the Port Mafia as its feared young killer. Akutagawa takes the hero's seat he never held in the main timeline, pushed there by a desperate effort to save his sister Gin, and he works beside Sakunosuke Oda, who is alive in this version of the city. This manga adapts Kafka Asagiri's BEAST light novel.