After the noble Kappa Kingdom falls to the Otter Empire, the Kappa prince Keppi loses much of his power and becomes helpless against the unseen Kapa-zombies. These zombies plague the world, and are the creations of the Otters and manifestations of people's deepest desires. With no other choice, Keppi must rely on three young boys: Kazuki Yasaka, who must carry a box with him wherever he goes; Enta Jinnai, Kazuki's childhood friend; and Tooi Kuji, a delinquent and a school truant. By having the mythical organ called a shirikodama removed from them, the boys are able to become Kappa themselves and fight the Kapa-zombies. However, to defeat them, the boys must connect with each other via their minds, bodies, and—most importantly—secrets. As the Kappa Kingdom relies on these boys, they must r