How do you fight a war without weapons? You use cunning, deceit and imagination. In fact, you fight dirty - just as the British were compelled to do in the summer of 1940 when faced with an imminent German invasion. During World War II, black propaganda, subversive rumours and outright pornography were all used by a secret Whitehall department to undermine the morale of the seemingly invincible Nazis. As the war progressed so did this extraordinary unit's operations. Its techniques became more sophisticated, its reach longer, and it went on the offensive. And the same mix of disinformation, ridicule and smut would eventually be employed against the Japanese.