After the Otter Empire conquers the aristocratic Kappa Kingdom, the Kappa prince Keppi loses most of his might and becomes defenseless against the invisible Kapa-zombies. These zombies, which are the Otters' creations and embodiments of people's darkest desires, afflict the planet. Keppi has no option but to rely on three young boys: Kazuki Yasaka, who must carry a box everywhere he goes; Enta Jinnai, Kazuki's lifelong pal; and Tooi Kuji, a delinquent and a school truant.