In 1941 a secret British radio station called on Germans to rise up against Hitler. Run by German exiles, it was explicitly left wing. The station’s target audience was “the Good German”. Its broadcasts were serious and idealistic: a ray of light amid totalitarian darkness. They were also a complete flop. With Nazi propaganda rampant, and Hitler’s armies seemingly invincible and on the march across Europe, few bothered to listen in. It was at this point that Britain’s wartime intelligence services tried a more radical approach. That summer, a talented journalist called Sefton Delmer was given the job of beating...