Potomas the Hippo harbors a secret: he can speak like a human, though only the young children Maki and Toshi realize this. He also wears a bright green T-shirt featuring a giant letter "P," which is supposed to be nicely inconspicuous. Momentarily forgetting to keep quiet, Potomas speaks to a child he has just saved from drowning. Rumors soon spread about a talking hippo, and Potomas is forced to go on the run.