What if the Port Mafia's hellhound, Akutagawa Ryunosuke, had joined the Armed Detective Agency? Likewise, what if Nakajima Atsushi belonged to the Port Mafia? In the parallel world of Bungo Stray Dogs: Beast, fledgling detective Akutagawa's desperate mission to rescue his younger sister from the Port Mafia's clutches brings him face-to-face with the organization's fearsome "White Reaper", a.k.a. Nakajim a Atsushi. What does fate have in store for the two beasts in this alternate tale of darkness versus light? Adapted from the light novel of the same title by Kafka Asagiri.