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. They are successful, but the act of "Sarazanmai" to transfer the shirikodama to Keppi exposes Kazuki's secret to the group: that he cross-dresses as the idol Sara Azuma. In a post-credits scene, police officers Reo Niiboshi and Mabu Akutsu extract the desires of a man they are questioning.

Enta, who has a crush on Kazuki, wishes to earn a dish so that Kazuki will return to playing soccer with him. Reo and Mabu extract the desires of Otone's boyfriend (who, unbeknownst to her, is a serial killer targeting women) and turn him into a zombie. The group defeats the zombie, exposing the secret of Enta's crush, and causing Otone to forget her boyfriend ever existed. In a post-credits scene, Enta visits Haruka in a hospital, who is revealed to be a wheelchair user.