By December 1944, Britain was denuded of fighting troops. Every man capable of handling a rifle was needed at the front on the Continent. Behind them they had left the lame, middle-aged soldiers or unfit men, many of them engaged in guarding the 250,000 very fit young Germans who were now scattered throughout the British Isles in their POW cages. Even the most determined Nazis among them knew there was no escape - so why try? But since September 1944, when the first German paratroops had started arriving at the cages in large numbers, the mood of these "tame" POWs had begun to change. They had become more truculent and had set up networks of hard-core Nazis and fighting men ("the blacks", as they were called by their British guards), who really ruled the camps behind the backs of the British. For the German High Command in Berlin had infiltrated their agents into all the most important camps in the UK, and it was their task to plan a mass breakout which would coincide with the start of Hitler's last great offensive in the West - the Battle of the Bulge. But this would be no ordinary escape - it was one with a strategic purpose, for the plotters planned no less than a march on London in a determined attempt to capture the capital of the British Empire. This account of the episode is a tale of double-dealing and treachery, which caused intense alarm at that last Christmas of World War II, and which ended in murder and retribution.