English commando Geoffrey Carter is chosen to undertake a dangerous mission to a small town near Calais, where the Germans have an important munitions factory. Carter kills two German sentries during the trip, after which he hides at the farm of Monsieur and Madame Bonnard. There Carter overhears German sergeant Conrad Block pressuring the Bonnards' pretty daughter Odette to become his "housekeeper." Odette spurns his advances, but cannot afford to be unkind to him, for he knows that she illegally keeps a goat to provide milk for her orphaned nephew. The next morning, Bonnard and Odette find Carter in the barn, but because he hides near the goat, keep his presence a secret when a German patrol searches for him. Once aware that Carter is an English commando, Odette, bitter because her brother Pierre was killed in the British attack at Oran, wishes to turn him in. Bonnard insists that they help him, however, and tells him that he can impersonate Pierre, who is known to be dead by the townspeople but not by the Germans. After being instructed in Pierre's habits, Carter goes to town, where he meets with Pierre's best friend, Jacques Grandet. Jacques works in the munitions factory and tells Carter how to distinguish it from the four phony buildings erected by the Germans to fool RAF bombers. The factory is surrounded by fields owned by Bonnard, his miserly brother Maurice, the Widow Grelieu and her three daughters, and Monsieur Danton. Carter persuades the land owners to create a fire break so that he can set a fire around the factory that night, thereby creating a target for the RAF's attack. Carter is confronted by Block, but uses Block's fraternization with Odette to get him in trouble with Commandant Hauptmann. The commandant orders Block to prove his charge that Carter is the commando, which Block does by gently persuading Madame Bonnard to show him photographs of the real Pierre. The Bonnards are arrested, and despite Odette's pleas, Bonnard remains determined to make any sacrifice necessary to defeat the Nazis. Hauptmann tells Odette that he will be lenient if she locates Carter, but after she helps to have Carter, Danton and the Grelieu women arrested, he orders that her parents be executed by a firing squad. Overwhelmed by remorse, Odette questions Maurice about Carter's plans and learns about the impending raid. Meanwhile, Carter is interrogated by Hauptmann and tells him that he planted a bomb in the factory, which causes Hauptmann to send in German soldiers to investigate and evacuate the building. Soon after, Odette arrives at the jail where Carter and the others are being held and frees them after killing Block. The conspirators then race to the fields, which they burn just in time to alert the RAF. The bombing mission is successful, especially as the factory is full of German soldiers rather than French workers. Odette rushes home to her nephew and asks Carter to take him to England. Carter, who must first rendezvous with English troops in Calais, offers to take Odette also, but she insists that she must stay and fight for her country. Carter assures her that soon he and many others like him will return, and Odette replies that many more like her will be waiting.