As Britain entered its second winter of World War II, nightly German blitzes rained fire on its cities and the threat of invasion had not yet passed. Britain stood very much alone. Yet wartime recruit and Oxford University professor, J.C. Masterman, had the confidence and foresight to predict a time when the tables could be turned against the Nazis. Since the outbreak of war, the British Security Service MI5 had been collecting a group of double agents. The Germans appeared to trust these spies and pressed them for more information. This presented an enormous challenge for MI5: how to preserve the credibility of their doubles without giving away vital war secrets? In a secret memorandum of 1940, Masterman presented an amazing solution. Crowdy’s new book reveals the content of the now-declassified memorandum and explores to what extent the Allies were able to realize Masterman’s plan to pull off an elaborate hoax on Hitler.