Take 20 men, or thereabouts. Assign them roles - make half of them prisoners, and half of them guards, and give the guards the task of keeping control among the prisoners. And watch things escalate.
Das Experiment, the movie we watched on Sunday, took this as its basic premise. It added a few features - a sympathetic protagonist, a suicidal and almost incidental heroine. Incredibly gripping movie. And all the scarier because there's a true story behind it all.