The Doctor and Leela arrive on Earth in what appears to be an industrial complex. While Leela is captured, the Doctor discovers that it is the site of a sociological study in which different societies in different time periods are recreated by conditioning volunteers into believing they are different people. Their goal is to learn why previous alien conquests of Earth had failed by determining the essential strengths and weaknesses of the human race, then selling the information. Leela encourages the test subjects to fight their captors, and the Doctor shuts down the machines in the control centre.