Some two million Cambodians were murdered between 1974 and 1979, victims of Pol Pot's fanatical Khmer Rouge regime. Close to 20,000 were tortured and executed at Security Office 21, or S21, a notorious detention center in the capital Phnom Penh. Only seven people survived S21; three are alive today. In the haunting S21, The Khmer Rouge Killing Machine, veteran Cambodian documentarian Rithy Panh (himself imprisoned by the Khmer Rouge as a child) persuades two of S21's surviving victims and several former guards to meet on the site of the old prison, now a memorial museum, and asks them to confront their terrible past – and each other.