A kid who looks like the thing everyone mocks him for discovers he's actually built for something nobody else can do. The mountain isolation, the swimming, the trip to Tokyo as the carrot, this isn't setting up a redemption arc, it's setting up a kid learning his own weirdness is his weapon. By the time Sanpei gets in that water, you're not watching him prove something to his classmates anymore.
Sanpei is a lazy, cheerful seven-year-old who lives with his grandfather in the mountains and cares about little besides his school lunches. His classmates tease him constantly for looking like a kappa, but it barely dents his good mood. When his school announces a swimming contest whose winner gets a free trip to Tokyo, Sanpei decides to compete, hoping to see his mother, who seems to have been working in the city for as long as he can recall. The trouble is that he swims poorly, so he slips off to a nearby lake to practice where no one can see. A strong undercurrent drags him under, and he drowns, only to wake up inside the magical world of the kappa. There he befriends a kappa child named Gartaro and starts to learn where his mother really is and why his father vanished. The film is a supernatural adventure built around a small boy's journey into the realm of the kappa.
