The stage is Asakusa. One day, three 2nd-year middle school students — Kazuki Yasaka, Tooi Kuji, and Enta Jinnai — meet a mysterious kappa-like creature named Keppi, who forcibly takes their shirikodama (a mythical organ kappa steal through a person's anus) and transforms the boys into kappa.

Keppi informs them, "If you wish to retur...