What if the Port Mafia's attack dog, Ryuunosuke Akutagawa, had joined the Armed Detective Agency? Likewise, what if Atsushi Nakajima belonged to the Port Mafia? In the parallel universe of Bungo Stray Dogs: Beast, 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... aka Atsushi Nakajima.