In 1975-79, the Khmer Rouge waged a campaign of genocide on Cambodia's population. 1.7 million Cambodians lost their lives to famine and murder as the urban population was forced into the countryside to fulfill the Khmer Rouges' dream of an agrarian utopia.
In S21, Panh brings two survivors back to the notorious Tuol Sleng prison (code-named "S21"), now a genocide museum where former Khmer Rouge is employed as guides.
Painter Vann Nath confronts his former captors in the converted schoolhouse where he was tortured, though by chance he did not suffer the fate of most of the other 17,000 men, women and children who were taken there, their "crimes" meticulously documented to justify their execution.
The ex-Khmer Rouge guards respond to Nath's provocations with excuses, chilling stoicism or apparent remorse as they recount the atrocities they committed at ages as young as 12 years old. To escape torture, the prisoners would confess to anything, and often denounce everyone they knew - though their final sentence was never in doubt.