Following the devastating fall of the noble Kappa Kingdom to the Otter Empire, Prince Keppi finds himself weakened and powerless against the invisible threat of the Kapa-zombies that now roam the land, a creation of the Otters and a reflection of humanity's deepest desires. With no other options, Keppi must entrust the fate of the world to three unlikely heroes: Kazuki Yasaka, always accompanied by a mysterious box; Enta Jinnai, Kazuki's childhood friend; and Tooi Kuji, a rebellious truant. Through the removal of a mythical organ known as the shirikodama, these boys become Kappa soldiers capable of battling the Kapa-zombies. However, their success requires more than just physical ability. They must also establish a connection with each other, sharing their deepest secrets and strengthening their bond. As they come to realize the fragility and value of true connections, they must also expose themselves like never before in order to save the Kappa Kingdom and the world beyond.