At the end of the Second World War, nearly 200 British citizens were under investigation for assisting Nazi Germany. Many have remained notorious, but this study shows there was a much larger and more intriguing story below the surface. Drawing from original documents, eyewitness accounts, and intelligence files, Adrian Weale traces the course of British treason in World War II, from its roots in Oswald Mosley’s British Union of Fascists, through the war and subsequent investigations, up to the trial, imprisonment, and, in some cases, execution of the traitors. In fact, German efforts to cultivate British traitors were largely unsuccessful—not least, as this book reveals, because much of that effort was entrusted to a British Fascist-turned-double agent at work in the heart of the Third Reich.