The moment all the adults vanish into a nostalgia trap, Shin-chan stops being a gag machine and becomes the only thing standing between his family and a world that decided growing up was a mistake. Comedy doesn't disappear, it sharpens, because every joke now has weight behind it. A five-year-old saving his parents from their own arrested development is absurd enough to make you laugh. It's also the only thing that could make you care whether he succeeds.

A new museum dedicated to 20th-century nostalgia opens in town, promising adults a chance to relive their childhood. But its owners have other plans. The grown-ups begin acting strangely, and one morning they walk out on their children and head for the museum. Among the kids left behind is Shinnosuke, whose parents Hiroshi and Misae have fallen under its spell. Together with his friends, Shinnosuke sets out to bring his parents home. The museum's curators, running a project called Yesterday Once More, want to erase the future and keep everyone living in the past century forever, wiping away the adults' memories of being grown up. The kids must stop the scheme before the world slips back into the 20th century for good. The result is a comedy film built around family and the pull of nostalgia.