Sometimes a story’s immediacy requires no artifice on the part of the filmmaker: They simply need to be there to capture it. However, over reliance on your story’s ability to engage the audience without any assistance can often doom (or at least seriously damage) an otherwise worthy film; the new, extremely worthy, and yet somewhat slight documentary S21: The Khmer Rouge Death Machine is just such a film.

Serving more as a historical document than a piece of filmmaking, S21 brings two survivors of a Khmer Rouge prison in the suburbs of Phnom Penh back to the prison – now preserved as a museum – and has them talk through their experiences, along with a number of men who were once guards at the camp. As quickly becomes obvious, S21 was actually more of an extermination camp than a prison: during the 1970s genocide that killed over 2 million Cambodians, over 17,000 of them were tortured to death or simply murdered in S21; their stark, black-and-white photographs now line the walls in silent accusation.

Director Rithy Panh – who escaped from a Khmer Rouge labor camp in 1979 when he was 15, after four years of imprisonment – gathered his subjects in S21’s eerie, empty rooms for what could have, in more manipulative hands, turned into a therapy session. The prisoners (the only two survivors of the camp that Panh could locate) speak heart-breakingly about the constant torture, how they were forced to confess crimes against the regime they hadn’t committed, and denounce dozens of others who were innocent but whose names simply jumped to mind and were likely tortured and made to denounce innocents themselves before being executed. More terrifying, though, are the guards, who matter-of-factly reenact the casual brutality they’d inflict on the prisoners. Their mantra, familiar to soldiers caught in atrocities throughout the ages, is, of course, “But I was given orders.” The banality of the horror becomes apparent when one of the guards, describing one particularly arbitrary mass killing, talks about the stink that rose off the corpses which nauseated him at first, but after a time, “became normal.”

Where Panh goes wrong with the film is relying solely on these stoic narrators to tell their tales. This is not to say that the material needed tarting up, or dramatization, but the lack of context, of an outside voice, saps the film of desperately needed structure. Although the basics of the Cambodia genocide are fairly well-known, there is plenty more background information which would have helped explain how this particular cog in Pol Pot’s infernal machine came about. The film presents us with the human aftermath quite adroitly, but in the face of such determined refusal to accept responsibility, the narrative never quite coheres.

All criticisms aside, S21 is somewhat mindful of an eagerly-awaited museum exhibit on an interesting topic which, on first inspection, turns out to have been poorly put-together. If there were a wealth of fine documentaries to choose from about the Khmer Rouge and the demonic evil they unleashed upon Cambodia, then S21would fare less well. As this is not the case, the film remains an imperfectly realized yet nevertheless important look at the depths to which humanity can sink all too easily.

The DVD includes an interview with Panh.

Aka S21: The Khmer Rouge Killing Machine, S-21, la machine de mort Khmère rouge.