Sanpei is a lazy seven-year-old boy who rarely focuses on anything other than eating his school dinners. Although his classmates constantly mock him for resembling a Kappa, Sanpei remains a relatively cheerful boy, quite content living alone with his grandfather in the mountains. One day, Sanpei's school announces a swimming gala, with the top prize being an all-expenses-paid trip to Tokyo. Sanpei decides to enter the contest to get the chance to visit Tokyo and meet his mother, who appears to have been working there as long as he can remember. Unfortunately, Sanpei is not a strong swimmer, so he sneaks off to a nearby lake to practice in secret. He is quickly dragged underwater by a strong undercurrent and drowns, only to regain consciousness in the magical realm of the Kappa. Thus begins Sanpei's journey as he befriends a Kappa child named Gartaro and starts to learn the true whereabouts of his mother and the reason behind his father's disappearance...