After the Kappa Kingdom fell to the Otter Empire, Prince Keppi, now weakened, faces a threat from invisible Kapa-zombies the Otters created to embody people's deepest desires. Deprived of his former strength, Keppi seeks help from three youths: Kazuki Yasaka, who always carries a mysterious box; Enta Jinnai, Kazuki's longtime friend; and Tooi Kuji, a troubled student who is often absent.
Having their shirikodama—a mythical organ—extracted gives the boys the ability to transform into Kappa warriors who can fight the Kapa-zombies. However, to fight effectively, the three must align their thoughts, actions, and secrets. With the Kappa Kingdom's survival at stake, the boys are forced to open up in ways they never have before, learning that bonds can be both fragile and precious.