What if the Port Mafia's Hellhound, 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...?