Ben Macintyre greets me warmly in the hallway of his period home in a leafy part of Islington, in north London. I’ve come to talk to him about Redwood, his latest book, a gripping true story of cold-war espionage in Iran just after the Islamic revolution of 1979 that ended the shah’s rule and installed Ayatollah Ruhollah Khomeini as supreme leader.

He is ever so friendly and charming. He is also completely barefoot. I notice several pairs of shoes in a heap in Macintyre’s hall. “Do you want me to…?” I suggest weakly, gesturing to my scruffy size 10s.

“No, no, no. Not at all. You can leave them on. Nobody pays any attention to shoes in this house,” he says. His aerated toes are just a sign of a “chill” demeanour, as my teenage daughters would put it.

He leads me into his attractive frontroom, which is suitably bookish for a Cambridge-educated author who has sold 6.5 million copies of his works. Macintyre, who is also a columnist for The Times, specialises in writing deeply researched but pacy nonfiction about conflicts and, for the past 20 years, spies.

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His shelves are packed with books about figures such as Leon Trotsky, the Russian revolutionary, and about second World War clashes such as those in the Ardennes.

Maggie O’Farrell’s historical novel Land, set in post-Famine Ireland, lies on a footstool beside Farewell to Eden, Sebastian Faulks’s novel set in the British Mandate of Palestine. On an ottoman beneath the front window lies an old copy of Blood & Oil, by Manucher Farmanfarmaian, the shah’s oil minister, who later fled Iran when his wealthy family was targeted by Khomeini’s regime.

“I think you can make the case that the Iranian revolution was the single most significant political event of the latter part of the 20th century,” Macintyre says.

Its ramifications certainly reverberate today, from the six-month old war that Donald Trump, the US president, is waging against Khomeini’s clerical successors to the anti-regime demonstrators you see all over London, including, a half-hour earlier, three outside Angel tube station, not far from the author’s home.

Macintyre says that western countries, such as Britain, have always regarded Iran as part of their backyard and somehow believe they “have a licence” to interfere.

“Redwood, however, is also a story about one man’s very personal and secret problem. I don’t want to oversell it, but we might be living in a different world if it had been handled in a different way.”

The western powers, he believes, “might have lost the cold war” – or worse, he argues, the world could even have slipped into nuclear conflict.

Redwood was the code name given by MI6, the UK’s foreign intelligence service, to Vladimir Kuzichkin, a KGB officer based in Tehran in the years either side of the 1979 revolution.

Macintyre’s book charts how the Soviets, in league with local communists, had devised a plot to seize postrevolution Iran. It would have given the USSR access to Persian Sea oil and a clutch of warm-water ports that might have tipped the balance in the cold war.

Crucially, Kuzichkin was the KGB’s liaison with Iran’s pro-Soviet Tudeh party. What the Soviets didn’t know, however, was that Kuzichkin had some years earlier flipped to become a British double agent. He fed everything back to MI6 – his main handler was a Tehran-based British spy named Ian McCredie – which also told the Americans.

They in turn fed this intelligence to the Khomeini regime, which executed those involved and disrupted the Russian plot to depose it – a plot with such significant implications that the US had been secretly contemplating deploying nuclear weapons to prevent a communist takeover of Tehran.

Kuzichkin was driven by a disdain for Marxism and his belief that the Soviet system was corrupt and wrong. But he also had a more personal motivation: he was suffering from erectile dysfunction and was terrified that, if word got out, it would expose him to ridicule among his KGB colleagues.

The West, Kuzichkin calculated, surely had a medical solution to give him “power in the bedroom”. Of all the things that could have befallen him – exposure, imprisonment, execution – the thing he feared most was his own sense of inadequacy.

Macintyre’s book paints a sensitive picture of a principled but vulnerable man, who was later spirited to Britain, where he lived as an unhappy alcoholic, loathing himself for betraying his own people.

“Spy stories never have neat endings. Few defectors end up having happy lives,” says Macintyre, who has often told the story of how he was himself “tapped up” by MI6 while at Cambridge, before it realised he wasn’t spy material (“I can’t keep secrets”).

Kuzichkin eventually had a daughter.

“She’s sort of philosophical about her father’s life. She chooses to remember the good and forget the bad about him. Kuzichkin was such a human figure. He’s not a James Bond character. He was huge, funny, awkward, shy, vulnerable. He is the un-Bond, the reverse of macho but also tough in his own way. And so brave.”

It’s odd to think that, had Viagra existed in 1981, doing away with Kuzichkin’s motivation to work for MI6, the world might indeed have ended in nuclear Armageddon.

Kuzichkin was also motivated by the nature of his relationship with his father. As Macintyre riffs about his books, espionage and geopolitics, I notice photographs on the shelves behind the author of his own father, the late Oxford academic Angus Macintyre, who was a noted scholar on Daniel O’Connell.

Angus Macintyre spent most of his childhood in Ireland. His mother was linked to a prominent Anglo-Irish family based at Furness House, near Naas, in Co Kildare.

“It wasn’t exactly a stately home. It was a big, freezing old house. It was so uncomfortable and the food was awful. My father’s family there were into fox hunting, country pursuits, that stuff. My father loved all of that in one way, but in another he didn’t. He got away from it all and ended up in Oxford.”

In 1994, at the age of 58, Angus Macintyre was killed in a motorway crash near Preston, in northwest England, as he drove to his family’s Scottish rural getaway a few days before Christmas – also his son’s birthday. Ben Macintyre was barely in his 30s at the time.

“It was terrible, just terrible. I often wonder what Dad would have made of my books. He only ever got to see one of them – and, alas, he never got to see it published,” he says.

“He was a proper scholar. He was all about footnotes, academic rigour. I sometimes think he would have looked at my books and said, ‘This isn’t proper history, sunshine.’”

Redwood in fact has 40 pages of an index and explanatory notes. Maybe some of his father’s rigour rubbed off on Macintyre after all. He smiles at the suggestion.

Macintyre has written so much about spies that Britain’s intelligence services ask him to give their officers historical briefings. He once gave Blaise Metreweli, who last year became the first woman to head MI6, a lift home, after one of his talks, when she was at MI5, the UK’s domestic intelligence agency.

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Macintyre says his father, who heavily identified with his Irish heritage, was horrified to discover that his son had been approached by British intelligence while he was at Cambridge. The Irish, after all, have a very different impression of British agents from the 007 image that the English, especially, love to picture.

Winston Churchill used to say the British had an aptitude for spying. Does Macintyre agree?

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“We certainly have a fascination with espionage, whether that makes us any good at it or not. There is this British love of secrecy and theatricality. But me? I would have been terrible at it.”

Much better to tell riveting stories about it instead.

Redwood: The Untold Story of the Cold War’s Most Extraordinary Double Agent, by Ben Macintyre, is published by Viking on Thursday, September 10th