Wounded by a Russian bayonet, air ace and decorated hero Major Mohn arrives at Colditz as the new second-in-command. He announces that Hitler has heard of the successful escape and he is here to enforce discipline and ensure that there are no other escape bids. However he antagonizes all the prisoners - particularly Carter, whose letters to his wife he reads - and the Kommandant wonders if he should get rid of him.
Assigned as the new escape officer Carter devises a new plan to fool the Germans. He fakes escapes after which the supposed escapees are actually hiding in the castle, ready to make the genuine attempt at a later date. However, the Kommandant, aware that the chapel is being used as the main starting place for tunnels, closes it, trapping Player and Brent in the pulpit.
Brent and Player survive to see new arrival Larry Page, a supposed RAF pilot officer -- though his story does not seem to check out -- and Walker, an actual RAF man, picks a fight with him. However Page actually turns out to be a spy, a member of Special Operations but, with no military status, he risks being shot as a spy. Consequently, rather than escape to resume operations, he opts to stay in the camp posing as a P.O.W.
Player observes that there are three British commandos in the town jail. Since Hitler has ordered that commandos should be denied P.O.W. status and shot, Preston asks the Kommandant, a fair man, to take them under his jurisdiction. Mohn backs this suggestion but he has an ulterior motive as he predicts, accurately, that the British prisoners will help the commandos to escape, and he means to be there to intercept them.
Given that it was previously used as the starting place for a tunnel, Mohn is unhappy about the boarded up theatre being used again. Ullmann, however, senses that the predictable British will once more use it in an escape attempt and brings in maximum security provisions. In the event the larger obstacle turns out to be the French, who are also keen to use it as their plan of escape.
Tony Shaw, a much decorated fighter pilot, is captured and brought to Colditz. To the disappointment of the escape committee he has no interest in joining them, rather renewing his pre-war job as a teacher and taking part in education classes with librarian Porteous. However, when he discovers a locked room in the attic he concocts a plan to build his own glider and fly out of the castle.
It is 1944 and technically the French and Germans are no longer at war with each other, meaning that the French captives will lose their prisoner-of-war status and be liable for transportation to labour camps in Poland. When the town padre requests that the camp's choir sing in the local church during a visit by the bishop of Lodz, French captain Andre Vaillant, helped by organist Gerda, uses the occasion as a diversion for his escape.
RAF officer Jack Collins arrives at Colditz and does not endear himself to his fellow inmates. He is a con artist and a card sharp, who fleeces the gullible Brent. However he is more interested in playing cards with Kruger, a German guard, who ends up in his debt. In addition to getting Kruger to supply metal I.D. tags for the castle's civilian workers, Collins intends his own independent escape plan, making use of his knowledge of Germany when he was a pre-war businessman and looking to his Jewish clients to help him out of fear of discovery.
Three American prisoners - including Carrington - are brought in to join lone countryman Phipps. However they are given preferential treatment and access to American newspapers, a deliberate ploy to make the British believe that they are spying for the Germans. Preston learns from the senior American, Colonel Dodd, that the trio are on a secret mission to make contact with the Hungarian free government, but they realise that they are under surveillance for information and must thwart the eavesdropper.
The war is almost over with Hitler holed up in the bunker in Berlin. SS officer Berger comes to Colditz, looking for high profile prisoners or Prominente, who can be used as hostages to be taken as reprisals. One of these is the son of the American ambassador, who would clearly be a prize, so Colonel Dodd, laid up with flu, asks the British to help him escape.
Mohn is in command in the absence of the Kommandant. He learns from friends in the town that the war is going badly for the Germans and, if he is captured his high profile connections with the Nazi party will grant him little quarter. As a consequence he attempts to ingratiate himself with the prisoners, leading to a confrontation with Carrington and his dismissal by the Kommandant, whom he has tried to blackmail. With the liberation of the camp inevitable Mohn makes his own escape from Colditz.
As the American tanks approach, the German officers move their families into the comparative safety of the castle, but the S.S. takes command of its running. Carrington has been sentenced to death for threatening Mohn, leading the senior officers to incite their men into disobedience. At this stage, however, Shaw at last believes the time has come to launch his glider, taking Carrington with him.
The allies advance on Colditz and the inmates take shelter in the cellars as the castle is bombed by pilots unaware of its nature. The S.S. put up a spirited rearguard action on the ground as Carrington takes advantage of the confusion to escape towards the Americam lines. Fearing brutality from the Russian army the Kommandant agrees to relinquish his command to the British prisoners on condition that he and his men are delivered to the Americans and Preston agrees. Whilst the Kommandant accepts defeat with his customary dignity he and Preston both agree that in war ...