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.

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?

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?

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.

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.

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.

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.

The allied armies get closer to Colditz and there is unrest when a death sentence is passed against one of the prisoners. Colonel Preston starts to mobilise the British in preparation for a possible revolt. The kommandant receives some terrible news – and a visit from the SS.