Derec, who has amnesia, and Katherine have been transported to mysterious planet containing an experimental city entirely populated by robots. The only other human that had resided in the city was murdered several days before their arrival, which was by transport using a mysterious device called the Key of Perihelion. Because they wish to keep secret how they arrived in Robot City, the robots assume either Derec or Katherine committed the murder. Of course, a robot couldn't do so due to the First Law. Thus, Derec and Katherine must clear their names.