"Yajū Shain Tsukishima" centers on Tsukishima, a salaryman working at a trading company who harbors a dark secret: he is a werewolf. The story explores the duality of his life, balancing the mundane routines of corporate work with the primal instincts of his supernatural condition. Tsukishima struggles to maintain his humanity while grappling with the violent urges that arise during the full moon. His condition is kept hidden from his colleagues, creating tension as he navigates office politics, deadlines, and personal relationships while concealing his true nature.

The narrative delves into the psychological toll of Tsukishima's double life, highlighting his internal conflict between his human side and his animalistic instincts. His interactions with coworkers, such as the perceptive and supportive colleague Aoi, add layers of complexity to the story. Aoi becomes a key figure in Tsukishima's life, offering him a semblance of normalcy and understanding, though her growing curiosity about his erratic behavior threatens to expose his secret.

The manga also examines themes of identity, societal expectations, and the struggle for self-control. Tsukishima's werewolf condition serves as a metaphor for repressed emotions and the pressures of conforming to societal norms. As the story progresses, his secret becomes increasingly difficult to maintain, leading to moments of intense drama and action as he confronts both external threats and his own inner turmoil. The series blends elements of workplace drama, psychological thriller, and supernatural horror, creating a unique narrative that explores the intersection of humanity and monstrosity in a modern setting.