In a world where nostalgia and childhood clash with the harsh reality of adulthood, the story takes us to a suburban area of Japan where five-year-old Shinnosuke Nohara, the mischievous little kid, becomes the epicenter of a strange phenomenon. It all begins when a group of adults, dazzled by the opening of a theme park that recreates the popular culture of the Showa era, gets carried away by the euphoria of the "perfect life" of their childhood. However, this apparent celebration soon turns into a threat to the future, as the adults, obsessed with reliving their youth, decide to completely ignore their current responsibilities.
Meanwhile, Shinnosuke and his family are swept up in this collective madness. As the influence of the park grows, the parents begin to forget what it means to be adults, neglecting their jobs, obligations, and, most concerning of all, their own children. In an effort to save the family and everyone's future, Shinnosuke embarks on a wild adventure filled with laughter and absurd situations, aided by his friends and his quirky pet. Throughout his journey, themes of responsibility, the importance of growing up, and the crucial balance between childhood and adulthood are raised, all while bringing us more than a few smiles and reflections on the value of never forgetting the child within us.