In the summer of 1971, psychologist Philip Zimbardo turned the basement of Stanford's psychology building into a mock prison and assigned college students at random to play guards or prisoners. The film uses footage shot during the actual experiment, later interviews with the participants, and Zimbardo's own narration to trace how quickly the roles took over. Guards invent humiliations on their own initiative, a prisoner has a breakdown and has to be released, and a planned two-week study collapses after six days because the situation has become genuinely dangerous. Zimbardo, watching his own experiment slide out of control, admits he stopped acting as a researcher and started acting as a prison superintendent. The film pushes past the shock value of what happened toward the harder question underneath: whether the guards were unusually cruel people, or whether almost anyone would behave the same way inside a system built to produce that behavior. Participants reflect candidly, decades later, on what they did and why.