Adolf Hitler had a wide array of British sympathisers, some of whom were abusing their positions of power while plotting a fascist coup. This study of the treachery and spying of fifth columnists relies on recently opened MI5 files to show how the establishment often ignored the activities of aristocrats such as the Duke of Buccleuch or the traitorous pilot who worked at the Air Ministry throughout the war, while pursuing humbler members of society using false-flag and spycatcher operations.