The film is based on the true story of Special Operations Executive French-born agent Odette Sansom, who was captured by the Germans in 1943, condemned to death and sent to Ravensbrück concentration camp to be executed. However, against all odds she survived the war and testified against the prison guards at the Hamburg Ravensbrück Trials. She was awarded the George Cross in 1946; the first woman ever to receive the award, and the only woman who has been awarded it while still alive. (From Wikipedia, licensed under CC-BY-SA)
Whilst not the paciest of WWII espionage stories, it is certainly one of the most considered - and by a clever use of subtle staging and lighting/sound manages to demonstrate the truly appalling nature of the Nazi treatment of the Allied intelligence gatherers/saboteurs and their brave French associates during the war. Based on a real person, a strong, determined, Anna Neagle - in the title role - is parachuted into occupied France where, with Trevor Howard and Peter Ustinov, she works to help the locals survive the tyranny of their new masters whilst passing back vital information to Britain. She is captured, tortured and it is all presented to us in such an evocative manner as to be really quite affecting. Ustinov is not his usual buffoon; and Howard, though still with his stiff upper lip - portrays "Capt. Churchill" (whom the Nazis think may be related to you know who) with delicacy and style. It lacks the visual violence and gore of many similar stories, but that just adds to the potency.