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.

Toi begins to destroy his connection to Kazuki and Enta, but ultimately chooses to preserve it. Keppi fuses with Dark Keppi, with assistance from the group and a revived Reo and Mabu, and defeats Otter and the concepts it represents. Upon performing sarazanmai, Kazuki, Toi, and Enta foresee a potential future in which their connections to each other are eventually severed. In the aftermath, Toi is sent to juvenile detention for three years. Upon his release, he jumps off of the Azuma Bridge, but is rescued by Kazuki and Enta. The three happily run "off to the future" together.