Ryuunosuke Akutagawa, had joined the Armed Detective Agency? Likewise, what if Atsushi Nakajima 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. Atsushi Nakajima. What does fate have in store for the two beasts in this alternate tale of darkness versus light...?