Twenty-four male students, chosen out of seventy-five applicants, are randomly assigned to play prisoners or guards in a mock jail built in the basement of the Stanford psychology building. Psychology professor Philip Zimbardo runs the study, funded by the U.S. Office of Naval Research to understand friction between military guards and prisoners, and it is scheduled to run two weeks. Footage shot during the experiment shows guards inventing degrading routines and stripping prisoners of names in favor of numbers, while several prisoners break down, one crying uncontrollably, another developing a psychosomatic rash. Zimbardo himself, acting as prison superintendent, admits he stopped thinking like a psychologist and let the abuse continue until his girlfriend, a fellow researcher, objected. Five prisoners quit early and the whole study is shut down after six days instead of the planned two weeks. The film uses the original recordings and interviews to lay out exactly how fast ordinary students slid into their assigned roles, and why the results are still argued over decades later.