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.

Kazuki asks for Toi's help to kidnap Sara and so that he can take her place during a meet-and-greet that Haruka plans to attend. Toi's backstory is revealed through a flashback – after his parents died from suicide upon being conned into crippling debt, the family's soba shop was saved from closure through Chikai's intervention. When a yakuza came to collect the family's debt, he was shot by Toi, which was subsequently covered up by Chikai.

Keppi transforms a hospitalized Enta into a kappa so that he and Kazuki can recover the dishes. Reo witnesses Otter performing sexually-charged "maintenance" on Mabu's heart; when he intervenes, Otter appears in the form of Reo, and explains that he is a mirror of Reo's inner desires. Meanwhile, Chikai is shot and killed in a confrontation with the yakuza.