Walter G. Krivitsky was the first top Soviet intelligence official to defect and reveal his secrets in 1939. In England in 1940, he came very close to unmasking the Soviet network inside Britain’s intelligence services known as the “Cambridge 5” led by Kim Philby. Krivitsky thought that he would be safe in America, but he was unable to shed the dangerous secrets that he took with him. Stalin had to act quickly to protect his vast espionage network and there would be no escape from the Soviet assassination squad. In Stalin’s Secret Service is like a spy thriller with an unwritten ending, because the author couldn’t imagine his own death.