What if the Port Mafias 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 Akutagawas desperate mission to rescue his younger sister from the Port Mafias clutches brings him face-to-face with the organizations 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…?