Based on recently declassified MI5 files, this is the story of one of Britain’s greatest intelligence operators, Maxwell Knight (1900–1968) or ‘M’. From 1923, when he was recruited for MI5 by Sir George Makgill, the book follows Knight’s career through infiltrating Communist and Fascist movements in Britain during the 1920s and 1930s, the Second World War and the Cold War, and examines his particular talent for recruiting and training special agents.