Following the fall of the noble Kappa Kingdom to the Otter Empire, Prince Keppi of the Kappa loses much of his authority and finds himself powerless against the unseen Kapa-zombies. These zombies afflict the world; they are creations of the Otters and manifestations of people's deepest desires. With no other option, Keppi must depend on three young boys: Kazuki Yasaka, who is required to carry a box with him at all times; Enta Jinnai, Kazuki's childhood friend; and Tooi Kuji, a delinquent and school truant. By having the mythical organ known as a shirikodama removed from them, the boys can transform into Kappa to battle the Kapa-zombies. However, to defeat these enemies, the boys must connect with one another through their minds, bodies, and—most crucially—their secrets. As the Kappa Kingdom depends on these boys, they must reveal themselves in ways they never have before, all while learning that connections are fragile and truly precious things.