"Bungo Stray Dogs: Beast" is a spin-off of the main "Bungo Stray Dogs" series, presenting an alternate universe where key characters and their roles are reimagined. The story centers on Atsushi Nakajima, a young man with the ability to transform into a white tiger, who finds himself in a drastically different position compared to the original series. In this universe, Atsushi is a high-ranking executive of the Port Mafia, a powerful criminal organization in Yokohama. His life takes a turn when he encounters Ryunosuke Akutagawa, a member of the Armed Detective Agency, an organization dedicated to solving supernatural crimes and protecting the city.

The narrative explores the dynamic between Atsushi and Akutagawa, whose roles are reversed from the main series. Atsushi, now a ruthless and calculating leader, is driven by a desire to maintain order within the Port Mafia, while Akutagawa, a more idealistic and justice-driven individual, seeks to challenge the mafia's influence. Their interactions are marked by tension and conflict, as both characters grapple with their opposing ideologies and the moral complexities of their respective organizations.

The plot delves into the power struggles within Yokohama's underworld, with the Port Mafia and the Armed Detective Agency clashing over control and influence. Atsushi's leadership is tested as he navigates internal dissent and external threats, while Akutagawa's resolve is challenged by the darker aspects of the city he aims to protect. The story also introduces alternate versions of familiar characters, such as Osamu Dazai, who plays a pivotal role in the unfolding events, and other members of both organizations, each contributing to the intricate web of alliances and rivalries.

Themes of loyalty, morality, and the consequences of power are central to the narrative, as characters are forced to confront their beliefs and the impact of their actions. The alternate universe setting allows for a fresh exploration of the "Bungo Stray Dogs" world, offering new perspectives on the characters and their relationships. The story maintains a balance of action, intrigue, and character development, making it a compelling addition to the franchise.