Synopsis
After the noble Kappa Kingdom is defeated by the Otter Empire, Kappa prince Keppi loses most of his strength and finds himself powerless against the invisible Kapa-zombies that plague the world. These zombies, created by the Otters, are the embodiments of people's deepest desires. With no other option, Keppi must depend on three young boys: Kazuki Yasaka, who always carries a box with him; Enta Jinnai, Kazuki's childhood friend; and Tooi Kuji, a delinquent and school truant. When their mythical organ, the shirikodama, is extracted, the boys gain the ability to transform into Kappa and battle the Kapa-zombies. However, to defeat them, the boys must connect through their minds, bodies, and—most crucially—their secrets. As the fate of the Kappa Kingdom rests on these boys, they must reveal themselves in ways they never have before, all while learning that connections are fragile and truly precious.