The Spy and the Traitor: The Greatest Espionage Story of the Cold War. By Ben Macintyre.Crown; 368 pages; $28. Viking; £25.

ON A breezy summer’s evening in 1988, officers of Britain’s Secret Intelligence Service, also known as MI6, gathered in a north London garden to toast Russia’s humiliating retreat from Afghanistan. The spooks’ mood was almost light-headed. The collapse of the Soviet Union and the communist system were in sight; both came to pass within a few years.

The guest of honour at the party, Oleg Gordievsky (pictured), was himself a Russian. Lionised by his hosts, Mr Gordievsky was Britain’s greatest intelligence coup of the cold war. Once a senior KGB officer, he had worked secretly for MI6 for years. He provided the West with a steady stream of high-grade intelligence considered so valuable that it frequently landed on the desks of both Margaret Thatcher and Ronald Reagan. After the ghastly betrayals and disasters of the early cold-war period, the era of Kim Philby and other British defectors, Mr Gordievsky represented a redemption of sorts. The burly, intense Russian was as ideologically driven as Philby had been, and he probably did as much damage to the Soviets as Philby had done to the British and Americans.

His tale has been told before, not least in his autobiography, “Next Stop Execution”. But Ben Macintyre, a journalist who specialises in books about spies and derring-do, has crafted his story as a real-life thriller, as tense as John le Carré’s novels, or even Ian Fleming’s. Based on many hours of interviews with Mr Gordievsky himself, as well as with his MI6 handlers and KGB veterans, “The Spy and the Traitor” is a gripping reconstruction, even for those with only a cursory interest in the secret world. But then Mr Macintyre has some exceptional material to work with.

Most exciting is the account of Mr Gordievsky’s exfiltration in 1985. Recently promoted to be head of the KGB station in London, and thus an even more valuable source for his British masters, he was abruptly recalled to Moscow. Fears that his treachery had been detected were confirmed when he was hauled in for gruelling interrogations by KGB goons. Despite swallowing a truth drug, Mr Gordievsky bluffed for long enough to alert MI6’s Moscow station to his predicament.

A long-rehearsed but precarious plan swung into operation: to get him across the Soviet-Finnish border under diplomatic cover. It worked, due mainly to the quick thinking of the wife of one of the MI6 drivers, who changed her baby’s nappy on the car boot just above the stowaway spy to put the guard-dog at the border off the scent. So important was Mr Gordievsky’s escape that the good news was instantly relayed to Thatcher. It was the first time that MI6 had managed to smuggle an agent out of Soviet Russia.

It had been a renegade CIA officer, Aldrich Ames, who shopped Mr Gordievsky. On this occasion, the CIA was as derelict in overlooking the tell-tale signs of his disintegration (alcoholism, spending sprees) as the Brits had been with their own traitors. Mr Macintyre brilliantly weaves together the two men’s parallel lives on opposite sides of the world, until their careers intersected. Mr Ames was later sentenced to life in prison for espionage.

How important was Mr Gordievsky in the end? Mr Macintyre makes a plausible case that he was the West’s most vital human asset in the 1980s. Thatcher and Reagan valued him particularly for his psychological insights into the thinking in the Kremlin. He convinced Western leaders that the bellicosity of their Soviet counterparts was a consequence more of fear and weakness than of ideological conviction. The West concluded that it could moderate its own anti-Soviet rhetoric without any loss of face or territory. The subsequent thaw in relations, especially after Mikhail Gorbachev took over, led to the end of the cold war.

In the new stand-off with Vladimir Putin, Mr Gordievsky may still be a target for Russian assassins. He sacrificed everything, including his family, in his act of betrayal. “He is one of the bravest people I have ever met,” Mr Macintyre concludes, “and one of the loneliest.”