"Ossan Idol!" centers around a 35-year-old office worker named Kenta Yamada, who leads an unremarkable life until a chance encounter reignites a long-forgotten dream. In his youth, Yamada harbored aspirations of becoming an idol, but societal pressures and personal insecurities led him to abandon those ambitions. Years later, after a series of mundane routines and unfulfilling days, he stumbles upon an audition for a new idol group. Despite his age and lack of experience, Yamada decides to take a leap of faith and audition, driven by a mix of nostalgia and a desire to break free from his monotonous existence.

The narrative explores Yamada’s journey as he navigates the competitive and youth-dominated world of idol culture. His age and unconventional background set him apart from the typical idol candidates, creating both challenges and opportunities. The story delves into his struggles with self-doubt, societal expectations, and the physical demands of idol training. Along the way, Yamada forms bonds with other aspiring idols, each with their own unique motivations and struggles. These relationships highlight themes of perseverance, camaraderie, and the pursuit of dreams regardless of age or circumstance.

As Yamada progresses through the ranks, he faces skepticism from industry professionals and the public, who question whether an older idol can succeed in a field dominated by younger performers. However, his genuine passion and determination gradually win over fans and peers alike. The series balances moments of humor, heartfelt emotion, and introspection, offering a nuanced portrayal of the idol industry and the personal growth of its characters. Through Yamada’s journey, "Ossan Idol!" challenges conventional notions of age and success, emphasizing the importance of pursuing one’s dreams at any stage of life.