During the First World War, two French aviators of the Service Aéronautique, the aristocratic Captain de Boëldieu and the working-class Lieutenant Maréchal, set out to investigate a blurred spot found on reconnaissance photographs. They are shot down by German flying ace and aristocrat, Rittmeister von Rauffenstein, and both are taken prisoner by the Imperial German Army. Upon returning to the aerodrome, Rauffenstein sends a subordinate to find out if the aviators are officers and, if so, to invite them to lunch. During the meal, Rauffenstein and Boëldieu discover they have mutual acquaintances—a depiction of the familiarity, if not solidarity, within the upper classes that crosses national boundaries.
Boëldieu and Maréchal are then taken to a prisoner-of-war camp, where they meet a colorful group of French prisoners and stage a vaudeville-type performance just after the Germans have taken Fort Douaumont in the epic Battle of Verdun. During the performance, word arrives that the French have recaptured the fort. Maréchal interrupts the show, and the French prisoners spontaneously burst into "La Marseillaise". As a result of the disruption, Maréchal is placed in solitary confinement, where he suffers badly from lack of human contact and hunger; the fort changes hands once more while he is imprisoned. Boëldieu and Maréchal also help their fellow prisoners to finish digging an escape tunnel. However, just before it is completed, everyone is transferred to other camps. Because of the language barrier, Maréchal is unable to pass word of the tunnel to an incoming British prisoner.
Boëldieu and Maréchal are moved from camp to camp, finally arriving in Wintersborn, a mountain fortress prison commanded by Rauffenstein, who has been so badly injured in battle that he has been given a posting away from the front, much to his regret. Rauffenstein tells them that Wintersborn is escape-proof.
At Wintersborn, the pair are reunited with a fellow prisoner, Rosenthal, from the original camp. Rosenthal is a wealthy French Jew who generously shares the food parcels he receives. Boëldieu comes up with an idea, after carefully observing how the German guards respond to an emergency. He volunteers to distract the guards for the few minutes needed for Maréchal and Rosenthal to escape. After a commotion staged by the prisoners, the guards are ordered to assemble them in the fortress courtyard. During the roll call, it is discovered that Boëldieu is missing. He makes his presence known high up in the fortress, drawing the German guards away in pursuit. Maréchal and Rosenthal take the opportunity to lower themselves from a window by a homemade rope and flee.
Rauffenstein stops the guards from firing at Boëldieu and pleads with his friend to give himself up. Boëldieu refuses, and Rauffenstein reluctantly shoots him with his pistol, aiming for his legs but misses and accidentally (and fatally) hits him in the stomach. Nursed in his final moments by a grieving Rauffenstein, Boëldieu laments that the whole purpose of the nobility and their usefulness to both French and German culture is being destroyed by the war. He expresses pity for von Rauffenstein, who will have to find a new purpose in the postwar world.
Maréchal and Rosenthal journey across the German countryside, trying to reach neutral Switzerland. Rosenthal injures his foot, slowing Maréchal down. They quarrel and part, but then Maréchal returns to help his comrade. They take refuge in the modest farmhouse of a German woman, Elsa, who lost her husband at Verdun, along with three brothers, at battles which, with quiet irony, she describes as "our greatest victories". She takes them in and does not betray them to a passing army patrol. She and Maréchal fall in love, despite not speaking each other's language, but he and Rosenthal eventually leave from a sense of duty after Rosenthal recovers from his injury. Maréchal declares he will come back to Elsa and her young daughter, Lotte, if he survives the war.
A German patrol sights the two fugitives crossing a snow-covered valley. They fire a few rounds, but their commanding officer, hurrying to the scene, orders them to stop, saying the pair have crossed into Switzerland.
Château du Haut-Kœnigsbourg, which appears in the film.
