In 1943, having expended enormous resources on recapturing escaped Allied POWs, the Germans move the most determined to a new, high-security prisoner of war camp supervised by Luftwaffe Colonel von Luger. USAAF Captain Virgil Hilts and Scottish RAF Flying Officer Archibald "Archie" Ives are imprisoned in isolation in the cooler after an escape attempt. Hilts bounces a baseball against the cooler cell wall, as he plans another escape.
Meanwhile, the prisoners mount an audacious plan to tunnel out of the camp in order to break out 250 men, not only to escape to freedom, but also to draw German forces away from battle to search for fugitives, to the point that as many troops and resources as possible will be wasted on finding POWs instead of being used on the front line. Led by RAF Squadron Leader Roger Bartlett, "Big X" of the camp escape committee, and senior British officer Group Captain Ramsey, the men organise into teams. American Flight Lieutenant Robert Hendley is "the scrounger" who finds needed materials, from a camera to clothes and identity cards. Australian Flying Officer Louis Sedgwick, "the manufacturer", makes tools like picks for digging and bellows for pumping air into the tunnels. Flight Lieutenants Danny Velinski and William "Willie" Dickes are "the tunnel kings" in charge of the digging. Flight Lieutenant Andrew MacDonald acts as intelligence provider and Bartlett's second-in-command. Lieutenant Commander Eric Ashley-Pitt of the Royal Navy devises a method of spreading soil from the tunnels over the camp, under the guards' noses. Flight Lieutenant Griffith acts as "the tailor", creating civilian outfits from scavenged cloth. Forgery is handled by Flight Lieutenant Colin Blythe. The prisoners work on three tunnels simultaneously, calling them "Tom", "Dick", and "Harry". The work noise is covered by the prisoner choir led by Flt. Lt. Dennis Cavendish, who also serves as surveyor.
Hilts and Ives conceive an escape, but are caught and returned to the cooler. Upon release, Bartlett requests that Hilts use his next escape as an opportunity for a reconnaissance of the area immediately surrounding the camp; Hilts refuses but does assist the prisoners as a scrounger. Meanwhile, Hendley forms a friendship with German guard Werner, which he exploits on several occasions to smuggle documents and other items of importance to the prisoners. Soon, Bartlett orders "Dick" and "Harry" to be sealed off, as "Tom" is closest to completion. While the prisoners enjoy a 4th of July celebration arranged by Hilts, Hendley and 2nd Lt. Goff, the guards discover "Tom". Ives frantically climbs the barbed wire surrounding the camp and is shot dead. The prisoners switch their efforts to "Harry", and Hilts agrees to reconnoiter outside the camp and allow himself to be recaptured. The information he brings back is used to create maps to guide the escapees. Blythe discovers that he is slowly going blind due to progressive myopia caused by intricate work by candlelight; Hendley takes it upon himself to be Blythe's guide in the escape. The last part of the tunnel is completed on the scheduled night, but it proves to be twenty feet short of the woods. Knowing there are no other options, Bartlett orders the escape to go ahead. The claustrophobic Danny nearly refuses to go, but is helped along by Willie. Seventy-six prisoners get away, aided by an air-raid blackout, before the escape is discovered, when Griffith impatiently exits the tunnel in view of an investigating guard.
All 76 POWs attempt to flee though various parts of the Third Reich. Danny and Willie steal a rowboat and proceed downstream to a major port, where they board a Swedish merchant ship to safety. Sedgwick steals a bicycle, then rides hidden on a train to France where the French Resistance get him to Spain to safety. Cavendish hitches a ride in a truck, but is delivered to the authorities, discovering many other fellow prisoners recaptured. Hendley and Blythe steal a plane to fly over the Swiss border, but the engine fails, and they crash-land. Blythe is shot by German soldiers. As he dies, he thanks Hendley for getting him out. Hendley is recaptured. Hilts steals a motorcycle at a checkpoint, jumping a series of barbed-wire fences at the German-Swiss border to escape from German soldiers; he lands in the wire of the second fence and is recaptured. While waiting to pass through a Gestapo investigation checkpoint at a railway station, Bartlett is recognized by Kuhn, a Gestapo agent who had transferred him to the camp, but Ashley-Pitt sacrifices himself by killing Kuhn and letting himself be chased and killed by soldiers. The resulting confusion allows Bartlett and MacDonald to slip away, but while boarding a bus, MacDonald blunders by accidentally replying in English to a suspicious Gestapo officer. MacDonald is quickly apprehended, and Bartlett is recognized and recaptured by Untersturmführer Steinach, a SS agent who also had transferred him to the camp.
In mid-transport, many prisoners, including Bartlett, MacDonald, and Cavendish are invited to stretch their legs in a field, whereupon they are all shot dead on the orders of Adolf Hitler under the pretense that they were trying to escape. Hilts, Hendley and the others are returned to the camp. In all, 50 men were killed, 23 were caught and only 3 successfully escaped. Luger is relieved of command of the camp for having failed to prevent the breakout. Hilts is taken back to the cooler where he optimistically bounces his baseball against his cell wall, as he has done so before.
