Sanpei is a young, carefree boy who lives alone with his grandfather deep in the mountains. One day, while Sanpei is taking a nap on a makeshift raft floating down the river, he is mistaken for a Kappa. A group of 'fellow' Kappas, worried that he might be spotted by humans, pull him underwater and whisk him away to the underwater Kingdom of the Kappa. Upon arrival, Sanpei is soon discovered to actually be a human and is immediately sentenced to execution. Realizing the situation he is in, Sanpei somehow manages to convince the Kappa elders that Human civilization is much more advanced and that they could learn a great deal from humans if they allow him to live. Unable to trust Sanpei completely, the Kappa Elder removes Sanpei's bellybutton (as this is where they believe his soul to be located) as a guarantee before sending him back to the Human world. He is escorted by a young Kappa "exchange student" who strikes a frightening resemblance to Sanpei.