On June 6, 1944, 150,000 Allied troops landed on the beaches of Normandy and suffered an astonishingly low rate of casualties. D-Day was a stunning military accomplishment, but it was also a masterpiece of trickery. Operation Fortitude, which protected and enabled the invasion, and the Double Cross system, which specialized in turning German spies into double agents, tricked the Nazis into believing that the Allies would attack at Calais and Norway rather than Normandy. The story of D-Day has been told from the point of view of the soldiers who fought in it, the tacticians who planned it, and the generals who led it. But this epic event in world history has never before been told from the perspectives of the key individuals in the Double Cross System. These include its director, a colorful assortment of MI5 handlers, and the five spies who formed Double Cross's nucleus. The D-Day spies were, without question, one of the oddest military units ever assembled, and their success depended on the delicate, dubious relationship between spy and spymaster. Their enterprise was saved from catastrophe by a shadowy sixth spy whose heroic sacrifice is revealed here for the first time. Double Cross is a captivating narrative of the spies who wove a web so intricate it ensnared Hitler's army and carried thousands of D-Day troops across the Channel in safety.