"Nidaime wa Cosplayer" centers on the life of Rintaro Tachibana, a high school student who unexpectedly inherits his late grandfather's role as the head of a prestigious yakuza group. Despite his reluctance, Rintaro is thrust into a world of organized crime, where he must navigate the complexities of leadership while maintaining his secret identity. The series juxtaposes his dual life as a yakuza boss and an ordinary teenager, creating a blend of comedy, drama, and action.

Rintaro's life takes a further twist when he discovers his passion for cosplay, a hobby that starkly contrasts with his yakuza responsibilities. His dedication to crafting intricate costumes and participating in cosplay events becomes a source of both personal fulfillment and comedic tension, as he struggles to keep his two worlds separate. The series explores themes of identity, responsibility, and the clash between tradition and individuality, as Rintaro attempts to reconcile his inherited role with his personal interests.

Supporting characters include Rintaro's loyal yakuza subordinates, who are initially skeptical of his leadership but gradually come to respect his unique approach. His classmates and friends remain unaware of his yakuza ties, adding layers of secrecy and humor to his interactions. The narrative also introduces rival yakuza factions, internal power struggles, and moments of introspection as Rintaro grows into his role while staying true to himself.

The manga balances lighthearted moments, such as Rintaro's cosplay escapades and awkward social situations, with more serious undertones involving the challenges of leadership and the moral dilemmas of the yakuza world. Through its engaging plot and well-developed characters, "Nidaime wa Cosplayer" offers a fresh take on the yakuza genre, blending humor, action, and heartfelt storytelling.