Kazuki Yasaka, an eighth-grader residing in Asakusa, is transformed into a Kappa alongside his classmates Toi and Enta by a mysterious entity named Keppi. This occurs after they accidentally shatter a statue of a Kappa, the city's guardian deity. Keppi, who asserts he is the primary heir to the Kappa Kingdom throne, commands them to battle Kapa-zombies—bioarms manufactured by his adversaries—if they wish to regain their human forms. To eliminate these foes, they must produce a specific sound known as "Sarazanmai," which can only be generated when the trio is united; however, the three boys initially struggle to connect. Furthermore, each time they emit the sound, one of their secrets is exposed to the others! They eventually agree to take risks to gather the "Plates of Hope," bestowed by Keppi upon defeating the Kapa-zombies, which have the power to grant their wishes. Yet, complications arise regarding the plates...! As they fight the Kapa-zombies, their intense pasts and internal conflicts continue to surface. This is a sarcastic fantasy about three boys who feel disconnected from those important to them, as they come to understand the true significance of bonds.