When, early in 1940, an important Soviet defector provided hints to British Intelligence about spies within the country's institutions, MI5's report was intercepted by a Soviet agent in the Home Office. She alerted her sometime lover, Isaiah Berlin, and Berlin's friend, Guy Burgess, whereupon the pair initiated a rapid counter-attack. Burgess contrived a reason for the two of them to visit the Soviet Union, which was then an ally of Nazi Germany, in order to alert his bosses of the threat and protect the infamous 'Cambridge Spies'. The story of this extraordinary escapade, hitherto ignored by the historians, lies at the heart of a thorough and scholarly expose of MI5's constitutional inability to resist communist infiltration of Britain's corridors of power and its later attempt to cover up its negligence. Guy Burgess's involvement in intelligence during WWII has been conveniently airbrushed out of existence in the official histories and the activities of his collaborator, Isaiah Berlin, disclosed in the latter's letters, have been strangely ignored by historians. Yet Burgess, fortified by the generous view of Marxism emanating from Oxbridge, contrived to effect a change in culture in MI5, whereby the established expert in communist counter-espionage was sidelined and Burgess's cronies were recruited into the Security Service itself. Using the threat of a Nazi Fifth Column as a diversion, Burgess succeeded in minimising the communist threat and placing Red sympathisers elsewhere in government. The outcome of this strategy was far-reaching. When the Soviet Union was invaded by Hitler's troops in June 1941, Churchill declared his support for Stalin in defeating the Nazi aggressor. But British policy-makers had all too quickly forgotten that the Communists would still be an enduring threat when the war was won and appeasement of Hitler was quickly replaced by appeasement of Stalin. Moreover, an indulgence towards communist scientists meant that the atom secrets shared by the US and the UK were betrayed. When this espionage was detected, MI5's officers engaged in an extensive cover-up to conceal their misdeeds. Exploiting recently declassified material and a broad range of historical and biographical sources, Antony Percy reveals that MI5 showed an embarrassing lack of leadership, discipline and tradecraft in its mission of `Defending the Realm'. This book will be of interest to all students of history, international relations, espionage and civil, national and international security.