The Defector is the untold story of how the defection of KGB agent Oleg Lyalin led to the expulsion of more than a hundred Soviet spies from Britain.

At the heart of Lyalin’s story is a narrative entwined with lies, disinformation, Kremlin deception campaigns, paranoia, intelligence failures by the CIA and MI5, and a tangled love life.

Unravelling amidst a Soviet mission to plant fake Kremlin ‘defectors’ within British and American intelligence agencies, this story reveals how the secrets Lyalin divulged, 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, ultimately, helped destroy the reputation of the US agency’s head of counterintelligence, James Jesus Angleton.

Drawing on newly declassified intelligence files, dozens of interviews with spymasters, and told in full here for the first time by one of Britain’s leading commentators on national security, this story 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 tension 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.