In the French Alps during World War II, members of the Maqui, an underground French resistance group, attack a convoy escorting autocratic German General Bruger. Thinking that they have killed the general, the fighters return to their headquarters, a farmhouse owned by Marceau outside the town of Le Barthe. There, Allan Lemaire, an American OSS agent working with the French underground, and Lionel Durand, the group's leader, toast their victory. Later, Lemaire proposes to Lisette, another member of the group, and she accepts. Meanwhile, Bruger, still very much alive, goes to German headquarters at Le Barthe and castigates the commandant, Maj. Zaughoff, for his failure to apprehend the Maqui. Demanding reprisal for the ambush, Bruger instructs the major to round up and execute twelve villagers unless they provide information about the Maqui. The next day, Lisette goes into the village to talk to the priest about performing her marriage, and as she leaves the church, she is arrested by the Germans and taken to be shot with eleven other villagers. Upon learning of Lisette's arrest, Lemaire rushes to the field in which the prisoners are lined up, arriving just in time to see her executed. Sometime later, after the Allied invasion at Normandy, high-ranking German officers plot to assassinate Adolf Hitler, who orders the officers hunted down and executed. Soon after, Lemaire and Durand are sent to Paris to meet Michel Navarre, one of the underground's best operatives. In Paris, Navarre explains that their mission is to free one of the German officers involved in the coup who is now imprisoned in Paris. Navarre continues that the officer has promised the Allies information about the plans of the German high command, in exchange for breaking him out of prison. When Navarre mentions that the officer's name is Bruger, Lemaire experiences conflicting emotions between hate for the man who killed his fiancée and duty to the resistance. After obtaining a layout of the prison and a schedule of Bruger's daily prison activities from the general's French girl friend, Nicole, Lemaire, Durand and Navarre pose as French civilians working for a Nazi organization that repairs and builds installations, and approach the prison to explain that they are there to repair an electrical circuit beneath the manhole outside the front gate. The men, accompanied by several other resistance members, then climb down the manhole, where they lay charges of dynamite. On the day of the escape, as Durand conceals a gun in his jacket, the getaway car pulls up in front of the prison and Lemaire lights the fuse to the dynamite. When the tunnel explodes they gain access to the prison by convincing the guards to open the gates so that they can turn off the gas. Once inside, Durand pulls out his gun and orders the guards to disarm while Lemaire grabs Bruger and shoves him into the waiting car. After driving into the mountains, they switch cars and provide Bruger with a new set of identity papers, then proceed to a nearby abbey that serves as a safe house for the underground. To test if Bruger can be trusted, they demand information and he provides them with the location of camouflaged gasoline tanks used for refilling the German Panzers. Lemaire then asks the abbot to transmit the information over his shortwave radio to ascertain if it is correct. While they are at the radio, a coded message comes in disclosing that the Allies plan to invade the South of France in one week. Bruger listens to the code, and later surreptitiously copies it onto a piece of paper. Once Bruger's information is verified, they continue on to meet a contact who is to drive them to an airstrip where a plane will pick them up and fly them to London. At the airfield, however, they are ambushed by the Germany army and forced to flee on foot. Lemaire and Durand take Bruger to Marceau's farm, where Marceau reluctantly agrees to help them, even though he regards Bruger as a butcher and a murderer. Claude, a member of the resistance who was in love with Lisette, determines to avenge her death and so spreads the news of Bruger's presence throughout the village. Later that evening, a message comes over the shortwave radio informing them that Nicole was seen visiting the prison, leading Lemaire to suspect that Bruger set them up. Hoping to discover the truth, Lemaire decides to lure Nicole to Le Barthe by telling her that Bruger is sick. When Nicole arrives the next morning, Lemaire and Durand tell her that Bruger has died and the Maqui has claimed his body. Caught off guard, Nicole blurts out that the Germans promised Bruger safe passage to London, confirming Lemaire's suspicions. Just then Bruger appears and slips Nicole the coded message. As Lemaire and Durand are about to load Bruger and Nicole onto Marceau's truck, Claude, leading a band of Maqui, arrives and seizes Bruger. In the chaos, Nicole flees and drives to the village where she gives the coded message to the major and informs him of Bruger's peril. Taking the truck, Lemaire and Durand track Claude and the others to the site of Lisette's execution. As they arrive, the Germans, brought by Nicole, attack, prompting the resistance fighters to scatter into the hills for a face-off. Although Nicole is shot, Bruger manages to escape. The Maqui then alert the freedom fighters in the area. At headquarters, when the major informs Bruger that the code has still not been deciphered, the general humiliates him for his incompetence and, after boasting that he has discovered that the underground has been operating from Marceau's farm, orders the farm destroyed. After blowing up the farmhouse and killing Marceau, the Germans take Lemaire and Durand to town in order to execute them. As the firing line prepares to shoot, the major runs out of his office with news that the message has been decoded and discloses that the Allies plan to invade the South of France. Just as Bruger orders the major to transmit the information to headquarters, the assembled members of the Maqui attack. Lemaire chases Bruger into the local graveyard, and after shooting him, stands at the foot of Lisette's grave.
