Kappa no Sanpei

A boy named Sanpei lives with his grandfather in the mountains. While napping on a river raft, he is taken by Kappa who mistake him for one of their own. They bring him to their underwater kingdom, where he is discovered to be human and sentenced to death. Sanpei argues that humans have much to offer and is allowed to stay only after his bellybutton is removed as a guarantee. He is returned to the human world with a Kappa exchange student who looks like him. Sanpei then embarks on a journey to teach the Kappa, avoid a Death god, find his parents, deal with angry Tanuki, and recover his bellybutton.