Living in Asakusa and studying in the 8th grade, Kazuki Yasaka and his classmates Toi and Enta venture upon an unexpected adventure. Culpable of inadvertently destroying a statue of the city's Kappa, the trio is unexpectedly transformed into the same creatures by a mysterious creature called Keppi. Though, they come to learn that Keppi is the rightful heir to the Kappa Kingdom's throne.
If they ever wish to regain human form, all three friends must comply with Keppi's demands and face the Kapa-zombies. These creatures are bio-arms belonging to Keppi's adversaries. A single sound, "Sarazanmai," can be the key to their victory, but it can only be made once the trio unites. However, every time they make the sound, one of their secrets is divulged.
Despite the risks, the boys set out to retrieve the "Plates of Hope," which Keppi has promised to grant their wishes, but troubles soon arise with the plates. With each fight against the zombies, the boys' former confrontations and personal struggles come into view. Their sarcastic account slowly morphs into a tale of brotherhood, displaying the real significance of their bond.
