After accidentally breaking a statue of a kappa at a shrine in Asakusa, middle school student Kazuki Yasaka encounters Keppi, heir to the throne of the Kappa Kingdom. He, along with his classmates Toi Kuji and Enta Jinnai, enrage Keppi by calling him a frog; he steals their shirikodama, a mythical organ located in the anus that stores an individual's desires, and transforms them into kappas. Keppi informs them they must defeat a zombie by extracting its shirikodama in order to become human again...