The Hour Before the Dawn

The Hour Before the Dawn is a 1944 drama film directed by Frank Tuttle and starring Veronica Lake and Franchot Tone. It was based on the novel by W. Somerset Maugham.

Years later, when the Germans invade Poland and initiate what would become World War II, Jim is headmaster at a school and has fallen in love with a young Austrian woman, Dora Bruckman, who works for his sisterinlaw, May. He is unaware that Dora really is a Nazi agent sent to infiltrate Britain. She meets regularly with her supervisors in London, Mrs. Mller and Kurt van der Breughel, posing as Austrian refugees.Jim objects to Britain declaring war on the Germans, but his brother Roger joins the Royal Air Force. The Germans start their blitz over London, and Dora participates by signaling with the headlights of Mays car to the bombers. She is discovered by Mays son Tommy and explains herself by saying the lights had been forgotten on. ........

Source: Wikipedia