The basic plot is that a group of volunteer subjects are divided into two groups - prisoners and guards - for fourteen days. Quite cheesy as a premise, and predictably things escalate but the writers manage to make each small escalation believable in a way most movies don't. In most movies a long list of unlikely choices must be made to further the plot and any one likely event (calling 911) would make the movie end immediately. These writers actually manage to arrange events that each escalation is not only possible but probable. I won't include spoilers because if I had read the plot before hand I would have assumed the movie sucked.

The fact that it is in german with subtitles might have actually helped it for me. There isn't much dialogue, it isn't too central to the story, half the words are english anyway, and you can forgive the occasional bad line as a poor translation.

I hope I don't sound too queer on the subject, but if you do watch it and think things unlikely you don't know people well enough. The individuals are given stakes in their position, are robbed of feedback and thereby robbed of consequences. Once you get that ball rolling, what is the opportunity cost of breaking just one additional rule...