After the Kappa Kingdom falls to the Otter Empire, Prince Keppi loses his powers and faces the threat of Kapa-zombies, mysterious creatures born from people's desires. To combat this menace, Keppi enlists three unlikely heroes: Kazuki Yasaka, bound to a mysterious box; Enta Jinnai, Kazuki's loyal friend; and Tooi Kuji, a rebellious truant. By transforming into Kappa beings, the boys fight the Kapa-zombies by sharing their deepest secrets and forming unbreakable bonds. Through this perilous journey, they discover the delicate yet invaluable nature of true connections, essential to saving the Kappa Kingdom.