Murder at Colditz

In November 1939, two British intelligence officers, Mackenzie and Dalby, are involved in a dangerous mission. The plan is to collaborate with top Nazi generals in a plot to assassinate Hitler. But they've been double-crossed. The operation is aborted, but it is certainly not forgotten. It becomes apparent that somebody wants them dead - badly. Later on in the war, when Dalby is captured and sent to Colditz, an assassin manages to break into the legendary castle during a snow storm and shoots Dalby dead.
This mysterious, obscure event turns out to be of great significance, as Mackenzie later learns from no less than Winston Churchill. There is another war within the war going on, and Churchill has decided that Mackenzie is the best man for the job.