The exploits of Tommy Sneum, the Danish-born spy who died in 2007, made him a legend in espionage circles. But until now, the full extraordinary story of Sneum's action-packed career as a British-run spy has never been told.

Based on hundreds of hours of interviews with Sneum, Mark Ryan describes how Tommy made an incredible escape from Denmark in a battered old Hornet Moth aircraft - which he had to refuel in mid-air by climbing out on the wing. Later, he escaped from Denmark again - by walking across a treacherous frozen sea on which two of his companions died.

Tommy brought over precious intelligence about the Nazi radar installations in Denmark and their atom bomb - his reward was to be imprisoned in Brixton as a suspected double agent and threatened with execution.

He cheated the hangman - but it is only with the publication of this enthralling book that Sneum can be celebrated as, in the words of Professor R.V. Jones, Churchill's chief of scientific intelligence, 'one of the true heroes of World War II'.