The Hour Before the Dawn

The year is 1923 in England and General Hetherton is instructing his grandson Jim to shoot with a rifle. Unfortunately Jim's dog runs in the way and Jim accidentally shoots and kills him. The incident affects Jim deeply and he becomes a pacifist.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 sister-in-law, May. He is unaware that Dora really is a Nazi agent sent to infiltrate Britain. She meets regularly with her supervisors in London, Mrs. Müller 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 May's car to the bombers. She is discovered by May's son Tommy and explains herself by saying the lights had been forgotten on.Jim and Dora marry to prevent her from being sent back to Austria. Kurt plans to use Jim in an effort to convince the more influential English to consider capitulation. He sends a fake letter to Jim, asking him to join an effort to educate refugee children, a task Jim gladly accepts.Jim is freed from military service, provided he does work on a farm instead. All the farms reject him as a worker, since they deem him a coward. When visiting one farm, he stops a worker from being cruel to an animal, winning the respect of the farmer, Searle, who agrees to hire him.Soon May hears that her husband will participate in a secret operation at a nearby airfield, and Dora passes this news on to Kurt. Dora's next mission is to signal German bombers sent to destroy the airfield, which is located near the Hetherton home.Roger receives a list from Captain Atterley, a friend of his. The list contains the names of possible German agents who may attempt to sabotage the airfield, and Dora's name is on it.At their next meeting, Kurt mentions to Jim that the Germans might consider negotiating terms for peace with Britain. Jim gets suspicious, and tells Dora afterwards that Kurt spoke more like a German than a Dutchman. Dora's behavior and reaction makes him slightly suspicious, but he dismisses it.Jim talks to his influential friend, Sir Leslie Buchannan, who explains that the government is no longer prepared to negotiate with the Germans. Dora starts preparing for her mission, and pours gasoline over a hay wagon. Tommy again spots her and spoils her plan. She brings him into the house to do off with him, but then she hears the bombers arrive and rushes out to set fire to the hay.While Roger gets confirmation that Dora is a saboteur, Tommy escapes his captivity and runs to Jim, who learns of his wife's betrayal. When Jim arrives home to his house, Dora is packed and ready to leave. She confesses to being a spy and then tries to shoot him but fails. Instead Jim kills her, just before Roger arrives.After the experience, Jim has a change of heart and joins the Royal Air Force to participate in the fighting for his country.

Question: How does Dora attempt to signal the German bombers at the airfield?
Answer: Setting fire to hay.