Bill et Arnie Judlow (Jack Palance) sont frères jumeaux. Marié à Ruth (Barbara Lang ), Arnie, un ancien boxeur professionnel, tue un homme par jalousie et se retrouve emprisonné à la prison d'État de San Quentin. Son frère et sa femme vont s'unir pour l'aider à s'évader. Dans leurs préparatifs, ils sont repérés par un voisin qui travaille à la prison, Henry Nova (Harold J. Stone), ce qui va compliquer leurs plans.
After the establishing shot of Montluc prison, but before the opening credits, the camera rests on a plaque commemorating the 7,000 prisoners who died at the hands of the Nazis.
Stalag 17 begins on "the longest night of the year" in 1944 in a Luftwaffe prisoner-of-war camp somewhere along the Danube River. The story is narrated by Clarence Harvey "Cookie" Cook (Gil Stratton). The camp holds Poles, Czechs, Russian females and, in the American compound, 640 sergeants from bomber crews, gunners, radiomen, and flight engineers.
Teenager Jackie Knowles (Richard Attenborough) drives a getaway car in a robbery. He is captured and sentenced to serve three years in a borstal institution run by a sympathetic governor (Jack Warner). He befriends Alfie (Dirk Bogarde) and Bill (Jimmy Hanley).
During the First World War, two French aviators, aristocratic Captain de Boeldieu (Pierre Fresnay) and working-class Lieutenant Maréchal (Jean Gabin), embark on a flight to examine the site of a blurred spot on photos from an earlier air reconnaissance mission. They are shot down by a German aviator and aristocrat, Rittmeister von Rauffenstein (Erich von Stroheim). Von Rauffenstein, upon returning to base, sends a subordinate to find out if the aviators are officers and, if so, to invite them to lunch. During the meal, von Rauffenstein and de Boeldieu discover they have mutual acquaintances—a depiction of the familiarity, if not solidarity, within the upper classes that crosses national boundaries.
British Captain Fred Allison (Leslie Howard) bids farewell to his new wife, Monica (Margaret Lindsay), whom he has only known for six days, and sets out for the war. He ends up a prisoner of war (POW), tortured by the fact that his wife has not written to him since the early days of his two year captivity.
Sergeant James Allen (Paul Muni) returns to civilian life after World War I but his war experience makes him restless. His family feels he should be grateful for a tedious job as an office clerk, and when he announces that he wants to become an engineer, they react with outrage. He leaves home to find work on any sort of project, but unskilled labor is plentiful and it's hard for him to find a job. Wandering and sinking into poverty, he accidentally becomes caught up in a robbery and is sentenced to ten years on a brutal Southern chain gang.
The film opens with an image of a wooden toy horse. Gradually we observe that this is an assembly line in a prison, staffed by prisoners. They sing (La liberté, c'est pour les heureux = "Freedom is for the happy") as they work. Close-ups of two prisoners (Louis and Émile, the film's main protagonists) indicate they've taken a work tool. The prisoner next to Louis occasionally looks on, looking somewhat bored. After dinner everyone goes back to their cell. After feigning sleep during a guard's nightly rounds, Louis and Émile sing the title song as they resume a project of sawing off the prison window. Émile cuts himself, and Louis kindly mends the wound with a handkerchief. The window breaks free and they attempt to escape. Louis is able to get over the retaining wall, but Émile is not successful. Louis escapes, accidentally knocks someone off a bicycle, and rides off on the bicycle. Meanwhile we hear a chorus suggesting he's about to be captured (Ce sera bientôt fini = "It'll soon be over"). Louis heads into a village emblazoned with the words "Finishing Line" - the cyclist he knocked over was in a bicycle race, and Louis has won first prize.