Action takes place in May 1940. After having a disagreement with his new wife, Flight Lieutenant Simon Carter (David McCallum) sets off for a night mission over Germany but he is forced to bail out of his crashing aircraft. On the ground, he befriends a young priest but the Gestapo arrests them both. Carter escapes first from the Gestapo and then from the prisoner of war camp he is assigned to – from there he is despatched to Colditz.

The arrival of a new security officer – Haupman Ullman, leads to a tightening up of security. However the British contingent manages to eavesdrop on a meeting that will decide the strategic use of Colditz and they are surprised at what they learn. Meanwhile, the disillusioned German second in command (Michael Gough) is giving cause for concern with his behaviour, and when the Gestapo arrive, they spring a surprise on the Kommandant and his new security chief.

German guards are foiling too many escape attempts which leads Colonel Preston to believe that there is an informer working within the imprisoned population, so an inquiry is started to identify the suspect, but this sets off a chain of events which reveal the darker side of life in Colditz.

Dick Player comes up with an escape idea that would involve going through the disused theatre, but things do not go according to plan when it seems the French have the exact same idea. Mohn is still investigating the last British escape and he is beginning to get close to the truth – but can Simon Carter divert his suspicions in a psychological chess game?

A new prisoner - Collins seems determined to buck the system by going things alone and this causes friction with Simon Carter – all the more so because he is a card shark and is causing gambling fever - especially with George Brent, and with a German guard, Kruger who he intends to use for his own means.

The prisoners can now see battle activity from the upper reaches of the castle, but there is strong SS resistance and Colditz comes under attack. Phil Carrington makes a break for American lines, but the situation is very confused and there are many obstacles to pass before Colditz can be truly liberated.