Mio, the only child of wealthy parents, often feels isolated. One day, her dear Panda-chan falls into the river. Kenichi, a frail boy whose parents are seldom around, comes to her rescue from the other side of the riverbank. With courage, Kenichi saves Panda-chan and writes a letter to return the beloved toy, sparking a back-and-forth exchange of messages between the two children. Panda-chan dutifully carries their letters, traveling up and down the river.