In 1943, an American pilot, Lieut. David Halloran, is stationed in England near war stricken London. While boarding a bus on Hanover Street, he meets an elegant English woman when she jumps ahead of him in line to board a bus. He spends the afternoon having tea with her in a café, and promises to win the war for her, but she will only tell him she works in an army hospital and that she should not be with him. As they leave the café, bombs devastate the city and Halloran loses sight of the woman in the chaos. When he locates her in the wreckage, he kisses her and asks to meet at the same place the following week. Although she will not tell him her name, he feels strongly about her, but notices the wedding ring that had been hidden under her glove. Claiming it is “too late,” she walks away. The following week, Halloran returns to Hanover Street and, after a long wait, the mysterious woman arrives. The two spend the day at a country inn outside of the city and make love. Because he still does not know her name, he jokingly calls her Fred as he tells her he loves her. On his next flight, Halloran is anxious, which surprises his crewmen, Jerry Cimino and Martin Hyer, who are accustomed to hearing him joke on dangerous missions. When Halloran later makes sarcastic remarks during a briefing, his outraged commanding officer, Col. Ronald Bart, promises to make things hard for him. Halloran and Fred regularly drive to the inn, and their relationship blossoms. Eventually she reveals that her name is Margaret Sellinger and that she loves him. However, Margaret has a child, Sarah, and is married to Paul, a British intelligence officer who has problems at work. Paul discovers there is a double agent in his ranks who has been informing the Germans of the identities of British secret agents. His best agents dead, Paul must train a new spy, Lieut. Wells, for a mission to crack a safe in Gestapo headquarters in Lyon, France that is known to contain a list of the names of counterspies. Meanwhile, Halloran becomes increasingly serious during flights and cancels a mission on Christmas when he senses something is wrong with his plane. Another team is sent in his place, although no one can find mechanical problems on Halloran’s aircraft. Later that night, Halloran learns that the mission failed and the men who went in his place were killed. As retaliation for Halloran’s bad attitude during briefings and for his cancelling the Christmas day flight, Col. Bart “volunteers” him for an extremely dangerous mission, in which he will drop a British spy into Lyon, France. The plan is for Wells, working undercover as a German officer, to deliver papers to Gestapo headquarters, then return to crack the safe containing the list of German double agents and photograph the list. However, as the mission approaches, Paul Sellinger voices concern to his superiors that Wells is not ready for the task. He calls Margaret at the hospital, apologizes for being “unexciting” and expresses love for her. Although confused by her husband’s behavior, Margaret is forced to end the call when she is summoned to help with patients. That night, as Halloran prepares his plane for the perilous mission, Paul replaces Wells as the spy. During the flight to Lyon, Germans attack the plane, killing Cimino and Hyer, but Paul and Halloran manage to jump from the plane seconds before it explodes. After parachuting safely to a location twenty miles from Lyon, Halloran prepares to walk to the English Channel to find a way back to England. However, Paul stumbles and sprains his ankle and Halloran instead decides to help him get to Lyon. They hide in a barn for rest, and witness the shooting of a German soldier by a French woman. The French woman discovers Paul and Halloran, but helps, after they explain their mission, by giving them the dead soldier’s uniform and car. Halloran changes to the soldier’s uniform to disguise himself as Paul’s aide. In England, a British officer notifies Margaret that Paul’s plane has been shot down and refers her to Col. Bart for more information. She looks for Halloran at their usual spot on Hanover Street and becomes dismayed when he does not appear. Meanwhile, Halloran, having guessed that Paul has more knowledge than experience, asks why he replaced Wells on the mission and Paul explains that he wanted the opportunity to be a hero. They deliver the documents to the Gestapo headquarters, then return later to retrieve the list from the safe. A German soldier working there attempts to talk to Halloran but becomes suspicious when Halloran, who cannot speak the language, does not answer. When other German soldiers on duty near the safe realize they are spies, Paul and Halloran knock them unconscious. Paul manages to locate and photograph the list of double agents, and the pair barely escapes on a motorcycle as German soldiers chase them. Meanwhile, Margaret meets with Col. Bart, who mentions that Halloran was the pilot of her husband’s plane. Halloran and Paul return to the farmhouse where they received help earlier. While the French woman’s father secretly alerts the Nazis of their presence, she arranges for the French Resistance to help them return to England. As they wait, Halloran and Paul discuss the women they love, and when Paul shows him a photograph of Margaret, Halloran realizes that Paul is her husband. At that moment, Germans arrive in search of them, forcing Halloran and Paul to flee from the farmhouse. When they reach a bridge where Resistance members are waiting, Paul is shot while crossing and stumbles over the side, but Halloran runs back to rescue him. When they get to England, Paul is taken to a hospital to be treated for his gunshot wound. In the hospital corridor, Margaret finds Halloran and they profess their love for each other, but agree that Margaret must remain with her husband. Margaret and Halloran end their affair, but promise to think of each other whenever they drink tea.