Never told in full before, this is the account of how, in 1971, the defection of a KGB agent in London led to the expulsion of more than a hundred Soviet 'diplomats' from the UK.

Drawing on newly released case files, The Defector tells a startling story of Soviet plans to plant fake double-agents within British and American intelligence services, the paranoia that ensued, and how the actions of a genuine turncoat, the former KGB agent Oleg Lyalin, and the secrets he revealed led to one of the most dramatic and pivotal moments in the Cold War.

For Lyalin's defection to Britain not only discredited a previous KGB defector, Anatoliy Golitsyn, the darling of the CIA, but eventually destroyed the reputation of the Agency's head of counterintelligence, James Jesus Angleton. As Richard Kerbaj writes: 'There was a poetic irony in Golitsyn's loss of credibility. It came, as he had previously feared, at the hands of a KGB defector. Except Oleg Lyalin had not been sent by the KGB - he was running away from it.'

At the heart of Lyalin's story is a narrative entwined with lies, disinformation, Kremlin deception campaigns, intelligence failures by the CIA and MI5, and a tangled love life. Told in full here, for the first time, by one of this country's leading commentators on intelligence and security, it shows how the Soviet Union finally lost the Cold War.