As movies enter their silly season, "S21: The Khmer Rouge Killing Machine" arrives like a sobering splash of cold water. This devastating, elegantly simple documentary about the ravages of the communist regime in Cambodia during the 1970s testifies not only to human dignity and resilience but to cinema at its most intellectually honest and morally necessary.

In 1975, the independent state of Cambodia fell to the Khmer Rouge, an agrarian communist movement that had engaged that country in a civil war since 1970. For the next four years, the Khmer Rouge, led by Pol Pot, instituted a series of murderous purges throughout the country, ultimately taking nearly 2 million lives. S21, the main Khmer Rouge "security bureau" in the capital, Phnom Penh, was the main detention center of the regime, where about 17,000 men, women and children were tortured and killed. Only a handful survived.

One of those survivors, an artist named Nath, is the center of "S21," a gripping cinema verite account of his emotional and troubling reunion with his former guards and interrogators. Now a genocide museum, the bleak concrete barracks of S21 serves as the spare backdrop while Nath and several of his captors sift through prison records, photographs and artifacts of one of the most brutal genocides in history. With no narration and only a few titles explaining historical context, "S21" trains the camera on victims and victimizers as they tell their own unvarnished stories to each other and, indirectly, to the world. The result is a deeply moving, provocative meditation on cruelty and suffering, all the more effective for being so starkly rendered.

From Nath and a fellow survivor we learn of the unspeakable atrocities they and their countrymen suffered -- the arrests, the interrogations, the torture, the ritualized "confessions" of counterrevolutionary treason (even falling in love, one man explains, was considered a crime against the state). Their accounts of lying for hours with the corpses of fellow prisoners, of catching crickets in their mouths and being beaten until they spat them out, of being starved and humiliated, are wrenching. But perhaps even more painful are the narratives of the guards -- some of them recruited and indoctrinated as teenagers -- who impassively describe their methods of questioning and abuse. In the film's most breathtaking passages, the guards physically reenact their savage routines, their bodies unleashing memories that had been buried under years of twisted political rhetoric and rationalizations.

The most frightening and dispiriting aspect of "S21" may not be the atrocities themselves but the ease with which otherwise decent men were able to commit them and their resistance to their own accountability. As a study in human psychology, the film may strike viewers as distressingly relevant in light of recent reports from Iraq. But "S21" never makes such glib equivalencies, nor does it offer up easy catharsis or closure.

Director Rithy Panh, who was forced to work in Khmer Rouge labor camps at 11, has provided a vital historical record in the face of decades of denial (Khmer Rouge officials didn't admit to the genocide until last year, after one of them had seen this film). But on another level, Panh has done something more difficult in addressing the proper role of an artist in the face of unspeakable acts. That role, he seems to say through this compelling, heartbreaking film, is fulfilled by choosing simply to bear witness.

S21: The Khmer Rouge Killing Machine (101 minutes, in Khmer with subtitles, at the Avalon) is not rated. It contains adult material and images of death and torture.