"Bungo Stray Dogs: Hitori Ayumu" is a short anime series that serves as a spin-off to the main "Bungo Stray Dogs" storyline. The narrative centers on Ayumu Nakajima, a young boy who becomes entangled in the world of the Armed Detective Agency after being rescued by Atsushi Nakajima. Ayumu, who shares a name with Atsushi's childhood friend, is initially portrayed as a timid and reserved individual. His life takes a dramatic turn when he encounters Atsushi, who saves him from a dangerous situation involving a mysterious ability user.

The plot unfolds as Ayumu begins to interact with the members of the Armed Detective Agency, a group of individuals with supernatural abilities who solve cases and protect Yokohama from threats. Ayumu's presence introduces a new dynamic to the group, as his connection to Atsushi and his own latent abilities become a focal point. The series explores themes of identity, belonging, and the struggle to find one's place in a world filled with extraordinary powers.

Key characters include Atsushi Nakajima, whose protective nature and sense of duty drive much of the narrative; Ayumu Nakajima, whose growth and development are central to the story; and other members of the Armed Detective Agency, such as Doppo Kunikida, Osamu Dazai, and Ranpo Edogawa, who provide support and guidance. The series also features antagonists who challenge the agency, adding tension and conflict to the storyline.

The narrative is structured around episodic cases that highlight the abilities and personalities of the characters while advancing the overarching plot. Ayumu's journey is marked by his gradual integration into the agency and his efforts to understand his own abilities and purpose. The series maintains a balance between action, mystery, and character-driven moments, offering a concise yet engaging exploration of its themes and characters.