The riveting untold story of how the defection of a KGB saboteur in London led to the rescue of MI5 after a series of disastrous intelligence failures.

Drawing on newly declassified intelligence documents and dozens of interviews with spymasters, The Defector tells a startling story of a Soviet mission to plant fake Kremlin agents within British and American intelligence services, the paranoia that ensued, and how the actions of a genuine turncoat, the former KGB officer Oleg Lyalin, resulted in one of the most dramatic and pivotal moments in the Cold War.

Lyalin led MI5 to rethink its relationship with the CIA. And his defection discredited a previous KGB defector, Anatoly Golitsyn, the darling of the CIA, and ultimately destroyed the reputation of the US agency's head of counterintelligence, James Jesus Angleton.

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 national security, it reveals how during the darkest moments of the Cold War one of the West's greatest achievements transpired as a result of MI5's break with the CIA.

The disclosure of the inside story of this historic event also comes at a time when there is a renewed interest in the relationship between transatlantic spy services—from the intelligence they share or hold back to the way they respond to their political masters and stand up to threats from Russia.