The episode of the opportunistic valet of Britain's ambassador to neutral Turkey during World War II--dubbed Cicero for the eloquence of the top-secret material he appropriated from his employer Sir Hughe Knatchbull-Hugessen and sold to the Nazis--is a staple of intelligence lore. Yet this remarkable and sometimes comical story has often been recounted with little regard for the facts, most prominently in the popular film Five Fingers. Now,...