When their bombers are shot down over France, five RAF pilots are stranded in the countryside. Their leader, Paul Lavallier, a member of the Free French movement who escaped to Britain after the Germans sentenced him to death, determines that they must contact British Intelligence in Paris to help them escape. Planning to steal some civilian clothes to wear on their journey to Paris, the five break into a bar where they are discovered by a German soldier. After overpowering the soldier, they steal his wallet and split up, agreeing to meet at a cathedral in Paris. When the other soldiers find their injured comrade, they pursue the five, firing their guns into the foggy night. In Paris, the German soldier reports the incident to Herr Funk, the Gestapo chief, who realizes that the fugitives must be the missing pilots. Funk alerts his forces to look for the bank note, stamped with the mark of the German paymaster, that was stolen from the soldier's wallet. Soon after, the pilots regroup at the Paris cathedral. Baby, one of the pilots, has been wounded in the escape and warns Paul that he has been followed by a Gestapo agent. Entering the confession booth of Father Antoine, his boyhood priest, Paul asks for refuge and help in locating British Intelligence, an underground organization. Suggesting that they hid in the sewers under Paris, Antoine directs them to the crypt door that leads to the tunnels. To insure Baby's escape, Paul diverts the agent who has been following him and crosses the street to a nearby café. When the agent begins to follow Paul, the pilot knocks over his coffee and accidentally tears the sleeve of Joan, the barmaid. After excusing himself to wash his hands, Paul slips out the bathroom window and up the backstairs into Joan's room. Soon after, Joan enters her room to change her ripped dress and prays to St. Jeanne, her patron saint and the French heroine and martyr who raised the siege of Orleans against the British, for a new dress. Although she is shocked to find Paul hiding in her closet, Joan tells him that his pursuer recently arrested a British Intelligence agent in the café. Desperate to send word to Father Antoine, Paul tells Joan that the priest has asked him to give her the bank note so that she can buy a new dress. He asks Joan to notify the priest that she has received the money, and asks her to deliver a message, written in Latin, from him. After delivering Paul's message, requesting that Antoine visit the condemned spy in prison, Joan buys her dress. The priest visits the spy, but when he asks the man to name a contact for the pilots, the man thinks that it is a Nazi trick. When Father Antoine prays to God for help, the spy relents and provides the priest with the information before going to his death. Meanwhile, Paul, exhausted, has slept the night in Joan's room and upon awakening, he proclaims that France will never be conquered and bids her farewell. Paul proceeds to the church, where Antoine hands him a paper with the contact's name. Soon after, the Nazis, alerted by the Gestapo agent, enter the church and after Paul slips the paper into a Bible, they arrest him for lacking the proper documents and take him to Funk, who recognizes him as the missing flyer. Pretending to believe Paul's story that he has left his papers at home, Funk issues him a pass and releases him, hoping that he will lead them to the others. Arranging to meet Paul in Joan's room, Antoine gives him the address of Mlle. Rosay, a British Intelligence agent, and warns him that he is being followed. To circumvent the Gestapo, Antoine suggests sending Joan to meet Mlle. Rosay. Although Paul at first opposes the suggestion, Joan insists on helping and he gives her a note for Mlle. Rosay and a gift of a new musical clock to replace her broken one. On the way to meet Mlle. Rosay, Joan passes the shop in which she purchased the dress. A Gestpo agent, alerted by the bank note, steps from the door and follows her. The Gestapo arrive just as Mlle. Rosay greets Joan, but the two women escape out the back door. Later that night, Joan returns to her room, her hand injured in the escape, and Paul confesses that he has fallen in love with her. The next morning, Mlle. Rosay comes to Joan's room, bearing a map of the sewers, and instructs Paul and his men to meet her colleagues' boat in one of the sewer outlets at 2 a.m. that night. She tells Paul to call her if he is unable to lose the agent tailing him. As night falls, Paul shows Joan the medallion of Free France that he wears and promises to return and marry her when the war ends. After he leaves, Joan sees the agent waiting in the street and runs to warn Paul. When Paul phones Mlle. Rosay for help, he hears a German voice in the background, and realizing that she has been discovered, hangs up and sends Joan to deliver the map to the others while he tries to shake the agent. Joan arrives in the tunnels to find Father Antoine reciting the twenty-third psalm to the dying Baby. Meanwhile, the agent doggedly tails Paul, who after several unsuccessful diversions, ducks into a steambath. Turning up the steam to create a blinding fog, Paul finally overpowers the man and shoots him with his own gun. Too late to meet the boat, Paul leaves Joan a note that he has missed his appointment. As Joan is reading the note, she hears the Gestapo climbing the stairs and quickly stuffs the paper in the clock. When the clock stops playing music, Funk finds the note and deduces that Paul is hiding in the church. He offers to spare the pilot's life if Joan will lead him to the others, and she agrees if he will allow her to see Paul alone. At the church, Joan tells Paul that the others are still waiting for him. Knowing that her betrayal of Funk will cost her life, Joan sends Paul away, promising to wait for him. After Paul leaves, Joan leads Funk and his men on a labyrinthine tour of the sewer tunnels, delaying them until she hears the sound of Paul's motor boat speeding away. On the day that she is condemned to die, Joan, wearing the dress that Paul bought for her, is visited by Father Antoine. As she bravely faces death, he reassures her that she will live on in the heart of France. A volley of shots signifying Joan's death before the firing squad the ring out, while the planes carrying Paul and the others soar back to Britain and freedom.