It was a documentary about the prison camps under the Angkar regime in the late seventies. Victims (the few of them who survived) and prison guards met and talked about what happened. A recurring theme was the indoctrination of the prison guards who still until this day can't explain the cruelty the were responsible for. Some of the stories about the interrogations sound very much like 1984 by George Orwell, but for real. Horrible.
The film was at times dull and depressing but the stories faces made up for it. This film makes Schindlers List look like a comedy at times, much because it felt much more real since it was a documentary. The terror that was going on in Cambodia probably still happens in some communist and fascist states today to a certain degree.
"How often do you get the chance to stare directly into the face of evil? S21 gives you that chance, and what do you know - evil's face looks just like everybody else's." A review from Rotten Tomatoes
Site with some of the paintings by the painter who appears a lot in this documenary.