One of the German guards is found murdered, but fearing reprisals from the Gestapo, the Kommandant reports it as suicide. The Gestapo decide nevertheless to investigate but at the same time, Colonel Preston asks Simon Carter to look for a murderer among the prisoners and clues indicate that Simon may have his hands full.

A trio of Americans arrive at Colditz to join the solitary US inmate, Jim Phipps – but his suspicions are aroused when he sees the newcomers getting access to American newspapers and better provisions and despite the fact that there is a familiar face, the British wonder if in fact they are housing a German trained fifth column.

Mc Donald, a Scottish officer, receives a ""dear John"" letter and wants to escape desperately to put things right at home which puts the escape committee in a spot. Phil Carrington befriends the man and does what he can to help, but McDonald is tall and loud and stands out in a crowd – will anyone want to escape with him?

Colditz has a choir and they are asked by the local pastor to sing in his church for the Easter service. Captain Vaillant sees this as a means of escaping in order to avoid being transferred to Poland with the rest of the French contingent. Meanwhile a secret radio is bringing news of the war to Colditz.

The opening episode takes place in June 1940. Captain Pat Grant (Edward Hardwicke) is captured and taken to a camp where he immediately starts organizing escape activity. After an unsuccessful escape he is sent to Colditz.

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.

When a lorry drives into the Castle courtyard, Captain Grant seizes the opportunity to put a blitz escape operation into action.

Simon gets an idea to hide prisoners to effect a dummy escape which would then take pressure off any genuine escape, but Major Mohn is highly suspicious and starts to impose further restrictions on the prisoners and this impacts Simon's plan and puts the lives of 2 men in danger.

The bruised and unconscious body of submariner Lieutenant Dick Player is washed up on a beach somewhere in France. He is wearing plain clothes and speaks fluent German, which makes his captors believe he may be a spy. He escapes from the hospital where he is taken to recover but is recaptured and interviewed by the Gestapo. Player had once lived in Germany as the son of a diplomat and manages to get to speak to a childhood friend – now an important government official who offers Dick a chance to work with a plot to overthrow Hitler, but at a price.

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.

The town jail is being used as an overflow for Colditz prisoners and while Dick Player is there he observes an intake of three new prisoners – but these men are British commandos and Hitler has given instructions for all commandos to be shot. An escape plan is hatched using the best route – but Major Mohn is taking more than a passing interest in these commandos and has ideas of his own.

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.

Colditz gets a new senior British officer, Colonel Preston (Jack Hedley). (The previous senior British officer, Major Donaldson is referred to in this episode but never seen). On his arrival he witnesses a futile escape attempt by Flight Lieutenant Carter and when the Kommandant meets him, he believes Preston is a man of honour who he can deal with. The colonel's new orders that seem to be appeasing the Germans cause conflict and dissension in the ranks, but there is method in the colonel's madness, which could come unstuck when a Polish officer decides to escape.

Flight Lieutenant Carrington (Robert Wagner) joins the British contingent in Colditz and he is viewed with resentment, especially by Simon Carter (David McCallum) and after they have a scuffle that ends up with the American in solitary, Carrington announces that he wishes to write a book that examines the war from the German point of view. The Gestapo come to interview Carrington and grant permission, but do they trust Carrington – and does he trust them?

Dick Player believes he has an excellent escape plan, but the escape officer rejects the idea and the feeling is that there are too many good escape ideas being rejected which results in an impromptu escape by Simon which coincides with an inspection visit – however, Simon learns some valuable information and the original escape looks as if it's back on – but Colonel Preston lays down certain surprising conditions.

For the first time gunfire can be heard near Colditz. Lieutenant Colonel Dodd is dozed up with the flu and Phil Carrington acts as senior American Officer and he – with Colonel Preston has to handle a crisis when the Germans plan on moving the promenenti prisoners from Colditz – and there is bad news for the kommandant as the SS take an interest in Colditz.

Colonel Preston receives bad news from home and has to make a decision about his command. Meanwhile Dick Player has to go to the dentist in town and gets set to escape, but while there he finds out information which could assist in a mass escape. It all depends on being able to bribe one of the guards, Private Heller – but will he play ball?

A new doctor joins the German staff at Colditz and his stickling for rules causes problems with the prisoners, but then Simon Carter sees this as an opportunity to be court-martialled which means being sent to Leipzig where there is a better chance to escape, but things don't go as planned and soon Simon is in danger.

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?

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.

A new second-in-command is appointed to Colditz, but Major Mohn – a distinguished and much decorated soldier is determined to continue fight the war inside Colditz and Simon Carter whose broken ankle is not mending is prepared to meet him head on, but a psychological battle between them has consequences for the Kommandant and Colonel Preston, who is forced to appoint a new escape officer.

The escape is on and there is tension in the air and not everything goes according to plan – but as some escapees find out, getting out of Colditz is only one part of the battle – getting out of Germany proves to be much more difficult.Flt-Lt Carter has been caught trying to escape but has managed to pass on valuable information which has made a previous escape plan possible. Captain Grant has been relieved of his duties as Escape Officer to lead them.

The arrival of one of Britain's top fighter pilots - Squadron Leader Tony Shaw causes disagreements in the German staff as to how to handle such a specialist prisoner – but Shaw turns out to be more interested in studying English literature with fellow inmate James Porteous – however will Shaw settle down to a life of study while Simon Carter is badgering him to become the focal head of escapes?

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.

Even though the war is reaching a close with the sound of gunfire and bombing being heard, there are tensions in Colditz. The British prisoners become suspicious of Major Mohn's new attitude towards them and his trips to the town reveal problems that will cause him to make a big decision.