After a psychopath murders a man who was on his way to a job as a deputy sheriff in a small California town, he assumes the deputy's identity and arrives in town to start his new 'job'. What he doesn't know is that the recently appointed sheriff and his mistress are plotting to murder her evil husband—and blame it on the new deputy.

20 volunteers agree to take part in a seemingly well-paid experiment advertised by the university. It is supposed to be about aggressive behavior in an artificial prison situation. A journalist senses a story behind the ad and smuggles himself in among the test subjects. They are randomly divided into prisoners and guards. What seems like a game at the beginning soon turns into bloody seriousness.