Allowed unrestricted access to MI6 files up to 1949, Jeffery has constructed a detailed and compulsive narrative. He achieves this by making the story "essentially one of people".
Starting in 1909 when MI6 was a one-man band run by Mansfield Cumming (fond of disguises and gadgets, "C" was the prototype for Ian Fleming's "M"), Jeffery sketches the "eclectic and cosmopolitan mix" in this odd trade.
They range from novelist Compton Mackenzie, who ran the Aegean Service from an ex-royal yacht, to Dudley Clarke, whose arrest in Madrid in 1941 dressed as a woman (not very convincing) did not hinder his rise to the rank of brigadier.
Towards the end, a more ambiguous figure appears. Kim Philby condemned a would-be Soviet defector "as surely as if he had pulled the trigger".