A zoo acquires a new specimen: "Fritz", a biped from "Brecht's planet." Fritz is intelligent, and his keepers treat him with a mix of courtesy and disdain; he is kept in a display with another (presumably female) biped and the two are required to work for a living, transcribing tapes made by explorers to their planet. One day, Martin Naumchik, a human male, is visiting the zoo when his personality and that of the biped are interchanged. The switch is the unintended consequence of an experiment in time travel that takes place at another location. Can they come to terms with their new bodies and new feelings? Who is really who?