Demoniac (1957)

Gervais and his friend Bernard, two prisoners of war, manage to escape from the camp where they are held. Bernard is determined to take refuge with Hélène, his war grandmother, a woman with whom he has been carrying on a romance by correspondence and Gervais agrees to accompany him. Unfortunately, Bernard is killed by a train before they get to destination. Gervais then decides to present himself to Hélène who, never having seen Bernard, mistakes the newcomer for her war godson. Too weak to restore the truth, Gervais becomes Bernard and moves in with Hélène. The situation gets even more complicated as two more women get into Gervais'life and entangle him in their web, Agnès, Hélène's jealous sister, and Julia, Bernard's sister, who pretends to recognize him...