During World War II, the impenetrable Colditz castle in Germany was selected as the site for a POW camp. The Germans reason that, since Colditz has been virtually impossible to get into or out of for centuries, it is ideal for housing the most contentious allied prisoners. British officer John Mills doesn't agree, and leads an escape through a subterranean tunnel. Only three of the prisoners (Mills included) survive the escape; the next step is to get out of Germany itself. Colditz Story is based on the reminiscences of P. R. "Pat" Reid, the real-life English officer portrayed in this film by John Mills.
~ Hal Erickson, All Movie Guide