In World War II, the Germans transformed Colditz Castle into a high security prisoner-of-war camp called Oflag IV-C. Its purpose was to restrain those Allied prisoners who had attempted to escape from other Oflags and so Colditz housed various nationalities who were mainly British, Dutch, French and Polish. Among the British prisoners are Pat Reid (John Mills) and Senior British Officer Colonel Richmond (Eric Portman). Richmond is warned by the Kommandant (Frederick Valk) that "escaping is verboten" but Richmond has no intention of heeding this advice. All the prisoners are wary of Priem (Denis Shaw), the chief security officer, who is efficient and tenacious.