Maxwell Knight (1900–1968) was widely known as a naturalist, BBC broadcaster and author who maintained a collection of household pets, including bears, baboons, bulldogs, spiders, snakes and marmosets. Two of his books were illustrated by a young David Cornwell (better known now as John le Carré). But for thirty years he was also MI5’s most influential agent runner, masterminding long-term penetration of the Communist Party of Great Britain (CPGB) and helping to ensure that fascism never took hold in Britain. Le Carré was one of his staff.

As a child he wanted to be a zookeeper, vet or taxidermist. He served two years in the navy in the First World War, then worked in London as a civil service clerk, paint salesman, jazz player and games teacher. Anyone who shared his flats, including his three wives and