Rex Dexter decides to visit the ship's junk room and discovers a talking crow (once human) who tells of a trip he and his wife took to Mercury. They developed engine trouble and a giant from the planet tells them that he hates Earth beings, and he has a way to turn them into animals, then hunt them to the death. Rex and Cynde head for Mercury to confront this giant, which they do, turning the giant into a worm, which the crow eats, then Jan and Yna are restored to their former selves.