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 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.  This book will be of interest to all students of history, international relations, espionage and civil, national and international security.