Liddell was one of three brothers who all won the Military Cross during the First World War and subsequently joined MI5. He initially first served in the Metropolitan Police Special Branch at Scotland Yard, dealing primarily with cases of Soviet espionage, until he was transferred to MI5 in 1931. His social connections proved important because in 1940 he employed Anthony Blunt as his personal assistant and became a close friend of both Guy Burgess and Victor Rothschild, and was acquainted with Kim Philby. Despite these links, when Liddell retired from the Security Service in 1952 he was appointed security adviser to the Atomic Energy Commission, an extremely sensitive post following the conviction of the physicist Klaus Fuchs two years earlier. No other member of the Security Service is known to have maintained a diary and the twelve volumes of this journal represents a unique record of the events and personalities of the period, a veritable tour d’horizon of the entire subject. As Director, B Division, Liddell supervised all the major pre-war and wartime espionage investigations, maintained a watch on suspected pro-Nazis and laid the foundations of the famous ‘double cross system’ of enemy double agents. He was unquestionably one of the most reclusive and remarkable men of his generation, and a legend within his own organisation.