Based on a children's book of the same name. A kappa and his mother live by themselves in the river. He makes friends with the village children until one day a child drowned by accident due to his neglect. The village adults deemed the kappa family at fault and they were going to be eliminated until a plague hit the village. People started dying but the kappa family had a cure. By using the potion knowledge of his mother and the special mortar deep under water, together they were able to make medicine to save the villagers. But after the events, can either group forgive each other?