"Tatami Galaxy" centers on an unnamed protagonist, a university student in Kyoto, who reflects on his life choices and regrets while narrating his experiences. The story unfolds through a series of parallel timelines, each exploring a different version of his life based on the clubs he joins during his freshman year. These clubs range from the film club to the cycling club, each offering a unique path filled with eccentric characters and surreal events. Despite the varying scenarios, the protagonist consistently encounters Ozu, a mischievous and manipulative classmate who often derails his plans, and Akashi, a sharp and pragmatic girl who becomes a recurring figure in his life.

The narrative structure is cyclical, with each timeline resetting after the protagonist reaches a moment of disillusionment or failure. This repetition emphasizes his inability to break free from his self-imposed limitations and his tendency to blame external factors for his unhappiness. Throughout the series, themes of regret, self-discovery, and the passage of time are explored, as the protagonist grapples with the consequences of his choices and his idealized vision of the "rose-colored campus life."

The supporting cast includes a range of quirky and memorable characters, such as Jougasaki, a self-proclaimed ladies' man; Hanuki, a mysterious and enigmatic upperclassman; and Kaori, a cheerful and optimistic friend. These characters often serve as foils to the protagonist, highlighting his flaws and contrasting his pessimistic outlook with their own approaches to life.

As the story progresses, the protagonist begins to recognize patterns in his behavior and the futility of chasing unattainable ideals. The final chapters bring a sense of resolution, as he confronts his regrets and gains a deeper understanding of himself and his relationships. The series concludes with a poignant reflection on the nature of time, choice, and the pursuit of happiness, leaving the protagonist—and the reader—with a sense of closure and newfound perspective.