At Natzweiler concentration camp, captured British SAS and SOE agents were executed by lethal injection and their bodies burned, on the orders of Dr Erich Isselhorst and Dr Werner Rohde. This film follows the postwar investigation into their crimes, led by Major Bill Barkworth of the SAS and Vera Atkins of the SOE, who tracked down the men responsible for killing their own agents behind enemy lines. Using archival photographs, wartime documents, and location footage from occupied Germany and France, the film reconstructs how a small, largely unofficial team pieced together testimony and forensic evidence to identify perpetrators who had scattered or gone into hiding after the war. It follows the trail from the camp itself through interrogations and trials, showing the gap between the scale of the atrocity and the limited postwar mechanisms available to punish it. The result is less a courtroom drama than an account of dogged, improvised detective work carried out by people determined that colleagues who died in secret would not be forgotten.