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.

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.