Burned by napalm as an infant and adopted by the guardian spirit of a river, Numamushi spends the first years of his life catching frogs to eat and learning to shed his skin like his serpentine father. Then one day, a lonely man moves into the abandoned house next to the river, and curiosity for all things human awakens in Numamushi.

This mysterious man shares Numamushi's penchant for eating frogs but also carries the scars of war, a love of the written word, and knowledge of the secret history of the house and its river. When he begins teaching Numamushi to read and write, the growing friendship between them opens an unexpected path to metamorphosis–and healing–for them both.