London, 1935, and the Blackshirts of the British Union of Fascists are on the march. Disgraced army officer Hugh Clifton is recruited by MI5 to infiltrate the Blackshirts, but unwittingly becomes a hero of the movement. Drawn into street battles against communists he uncovers a plot to bring mayhem to Britain and undermine the government. He is being watched by the sinister fascist intelligence unit Department Z and decides the best way to beat them is to join them. Socialite Sissy is searching for a friend who vanished inexplicably and the two investigations become entwined. The mis-matched couple face danger and treachery as they unravel the facts, and Hugh must decide who to trust-and who to betray.
