The German POW camp, Stalag 17, exclusively populated by American sergeants, is overseen by sadistic commandant Oberst Von Schernbach and the deceptively avuncular sergeant Schultz. The inmates spend their waking hours circumventing the boredom of prison life; at night, they attempt to arrange escapes. When two of the escapees, Johnson and Manfredi, are shot down like dogs by the Nazi guards, Stalag 17's resident wiseguy Sefton callously collects the bets he'd placed concerning the fugitives' success. No doubt about it: there's a security leak in the barracks, and everybody suspects the enterprising Sefton — who manages to obtain all the creature comforts he wants — of being a Nazi infiltrator. Things get particularly dicey when Lt. Dunbar, temporarily billetted in Stalag 17 before being transferred to an officer's camp, tells his new bunkmates that he was responsible for the destruction of a German ammunition train. Sure enough, this information is leaked to the Commandant, and Dunbar is subjected to a brutal interrogation. Certain by now that Sefton is the "mole", the other inmates beat him to a pulp. But Sefton soon learns who the real spy is, and reveals that information on the night of Dunbar's planned escape.