In France, the Mother Superior of a convent, and the convent's gardener, Pierre Cabeau, work with the French underground to help escaping prisoners of war. When an American pilot named John is shot down over France, he is directed by the underground to the convent's chapel. His appearance there startles Sister Clothilde, a novice who dislikes men, but she informs Mother Superior of his presence. Because he is carrying important information from the French underground intended for the English government, arrangements are made for John to pose as the shell-shocked husband of a local woman, Madame Bouchard, and escape to England using forged papers. Before he can flee, however, Nazi Major Krupp breaches the cloister and enters the convent grounds, frightening Clothilde when he threatens to take several girls for "service" unless John is found. Clothilde, who has not been outside the convent walls since she was eight years old, unintentionally reveals John's presence to Krupp. Mother Superior later scolds Clothilde for her naivete, and advises the girl that as nuns, they are not exempt from the suffering of real life. Later, Pierre brings word that Mme. Bouchard has been arrested while John was waiting for her at the bridge. When German soldiers insist on searching the convent, Mother Superior refuses to admit them and is accidentally shot when they shoot out the lock on the door. Having understood Mother Superior's previous words to her, Clothilde meets John at the bridge and takes Mme. Bouchard's place, posing as "Louise Dupree." At a windmill operated by the underground, a passport photo is taken of Clothilde, and she and John are given instructions. While pretending that John is Clothilde's mute husband, the two make their way across the countryside by night, and arrive at an abandoned farmhouse after narrowly escaping death during an air raid. John is wounded during the attack, however, and Clothilde administers medicine which, along with food, has been hidden in the farmhouse by the underground. When John awakens in the morning, he finally recognizes Clothilde as the novice from the convent. Clothilde admits that she loathes men because of her father's infidelity, but when John speaks reverently of his wife and son, she sees the beauty of marriage. For the rest of their journey, Clothilde, with a new affection for John, ably assists him. Upon reaching the train station, they learn that an air raid has damaged the tracks and the train will not be running. Clothilde and John then go to a restaurant, but when a German soldier grabs her for some female companionship, John strikes him. Clothilde is recognized by Vitrey, the mayor of her town who has been searching for John, and he takes them hostage. John, however, knocks Vitrey out after wresting his gun from him, and they escape to a safe house. Madame Sarroux, the landlady, has arranged with Pierre, who is there, for Clothilde to work as a waitress in a boardinghouse, where John will eat and then hide in a storeroom until he hears the mechanized playing piano. According to the plan, John will then slip through a trapdoor into a waiting boat. Unknown to them, Nazi soldiers have tracked them to Sarroux's house. Clothilde and John escape to the boardinghouse, but Sarroux is unable to withstand the Nazi torture and reveals their destination. Clothilde is captured by Krupp at the boardinghouse, and, in exchange for John's life, makes a deal with Krupp to bring him the secret document. Krupp lets Clothilde go, trusting her nun's vow that she will return. John insists that for her own safety, Clothilde accompany him out of the country, but she demurs, and they bid each other a heartfelt farewell. John climbs through the trapdoor to the waiting boat, and Clothilde bravely returns without the papers to face Krupp. Although Clothilde maintains that she kept her promise to return, an angry Krupp declares that he will force her to "service" German officers as punishment. Appalled by this sacrilegious suggestion, Vitrey pulls his gun, and as a Nazi soldier struggles with him, the gun discharges and kills Clothilde.
