The cinema of genocide has understandably concerned itself with the stories of victims and survivors. Analyses of the bureaucracy of terror and of the perpetrators' psychology are far rarer. Rithy Panh's devastating documentary exploration of the Cambodian tragedy unites both approaches to harrowing, extraordinary effect. He builds his case from the concrete details of procedure, focusing on S21, the country's main security bureau. From 1975 to 1979, 17,000 prisoners were processed, tortured and executed there. Only three, it seems, are still alive. One is painter Vann Nath, reunited here with his former guards. Panh follows the painter's attempts to understand how and why they acted as they did, 'until there was nothing human left'. Dead-eyed, the guards recount their involvement in the 'mild', 'hot' and 'rabid' interrogation teams, read from the meticulous prison documentation and re-enact their daily routines. With great dignity, Vann Nath's interrogation shows how finally, with the normalisation of atrocity under a regime that sought to erase history and even time, 'all of Cambodia would have been enemies'. Asking the most profound questions about human agency, the film bears comparison with Shoah for its insights into the machinery and mindset of barbarism. It is dedicated, with heart-rending urgency, to 'remembrance'.