From 1975 to 1979 'Comrade' Duch was in charge of S 21, the security prison at the heart of Pnomh Penh where 12,380 people were tortured and executed, having confessed to imaginary betrayals of the regime. After his film S21, which brought survivors and executioners from the Khmer Rouge era together, Rithy Panh decided to film Duch in prison. During 300 hours of filming he confronts the man in charge of the campaign of extermination, tries to understand his personal history, his ideology, his methods. He talks to him about how he recruited his torturers, but also about his passion for numbers, for order. The process of confronting Duch every day, his cruelty, his evasions, his laugh draws Rithy Panh back to the past and the horrors of the Khmer Rouge era. Exhausted and despairing, he decides to tell his own story and that of his family. Against the evil of Duch he holds up the good embodied in the person of his own father, who believed in and fought for justice and education, and who perished in the Khmer Rouge genocide.