"Ojisan Kaizō Kōza" centers on the life of a middle-aged man named Ojisan, whose mundane existence takes a dramatic turn when he encounters a mysterious organization offering a radical self-improvement program. The series explores his journey as he undergoes a series of bizarre and often comedic transformations, both physically and mentally, under the guidance of the enigmatic group. The program promises to reshape his life, but the methods employed are unconventional, ranging from extreme physical challenges to surreal psychological experiments.

The narrative delves into Ojisan's struggles with self-doubt, societal expectations, and the pressures of aging, as he navigates the increasingly absurd demands of the program. Along the way, he encounters a diverse cast of characters, including fellow participants with their own unique motivations and quirks, as well as the eccentric leaders of the organization who seem to have their own hidden agenda. The series balances humor with moments of introspection, highlighting the absurdity of the self-help industry while also addressing deeper themes of identity and personal growth.

As Ojisan progresses through the program, the line between reality and illusion begins to blur, raising questions about the true nature of the transformations he undergoes. The story builds toward a climactic revelation about the organization's ultimate goal, forcing Ojisan to confront his own desires and fears. The series concludes with an open-ended resolution, leaving room for interpretation regarding the impact of his journey and the authenticity of the changes he has experienced.