Potomas the Hippo has a secret-he can speak like a human being, but only the young kids Maki and Toshi realize it. He also wears a bright green T-shirt with a giant letter "P" on it, but this 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.