Between 1939 and 1945, more than 70 Allied soldiers were convicted—mostly in secret trials—of working to help Nazi Germany win the war. In the same period, hundreds of British Fascists were also interned without trial on detailed evidence that they were working on behalf of Germany. Collectively, these men and women were part of a little-known Fifth Column: traitors engaged in espionage and propaganda work. Sifting through hundreds of declassified files, Tim Tate crafts here a thrilling account of how these spies almost succeeded, and of those who managed to thwart them.