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.

The group rescue Haruka after he is kidnapped by Reo, and are transformed back into human form. Keppi explains that Reo and Mabu are agents of the Otter Empire, who have been at war with the Kappa Kingdom for generations over the energy of the shirikodama; with the Kappa Kingdom now defeated, the Otter Empire has begun to extract the shirikodama of humans. In a post-credits flashback set during the Otter-Kappa war, Mabu sacrifices himself to protect Reo in battle.

Kazuki, Enta, and Keppi encounter Dark Keppi, the corporeal form of half of Keppi's shirikodama held by the Otter Empire. Otter, having learned of Reo's plan to use the dishes on Mabu, transforms Mabu into a zombie. Keppi transforms Reo into a kappa, who recovers Mabu's shirikodama and learns that Otter forced Mabu into giving up his connection with Reo in order to live. Mabu tells Reo that he loves him, causing his mechanical heart to explode. A grieving Reo attacks Kazuki, Enta, and Keppi, but is shot and killed by Toi, who wishes to use the dishes to bring Chikai back to life. Kazuki uses the dishes to save Enta's life, and Toi is absorbed by Dark Keppi.