In November 1960, Greville Wynne, a 41-year-old British businessman, sat down for a lunch that would change his life. His dining companion, Dickie Franks, revealed himself to be an officer of the British Secret Intelligence Service, also known as MI6, and asked Wynne for his help. An industrial sales consultant who regularly traveled through Eastern Europe and the Soviet Union representing British electrical and steel companies , Wynne was told it would be helpful if on his next trip, he could arrange for a meeting with a state committee in Moscow dedicated to developing opportunities with foreigners in science and technology, and report back on his conversations. Despite having no previous experience in intelligence work, Wynne was being recruited to serve as an MI6 agent.

Wynne agreed, and during his visit to Moscow the following month he wound up connecting with Oleg Penkovsky, a lieutenant colonel in the GRU (the Soviet Union’s foreign-intelligence agency) who was eager to leak high-level military information to Western powers. Penkovsky felt stunted in his career with GRU and expected that by helping the West for a year or two, he and his family could be relocated and build a better life, and that he would personally be showered with recognition and honor. Wynne went along, slightly concerned about whether Penkovsky was on the level and concerned about putting himself into a dangerous situation, kicking off what would be one of the most productive clandestine operations in Cold War history. Penkovsky’s information, and Wynne’s help in delivering it to British and American intelligence officers, would produce mountains of material, play a role in the Cuban Missile Crisis, and land both men in prison.

These events serve as the inspiration for The Courier , the new film starring Benedict Cumberbatch as Wynne and Georgian actor Merab Ninidze as Penkovsky, out in theaters on March 19. The film’s screenwriter, Tom O’Connor, found Wynne’s story of a nobody suddenly becoming a somebody compelling. “He just was an ordinary man who got thrust into this just extraordinary, life-altering situation that was going to define his existence forever,” says O’Connor. “The burden of that is hard to imagine.”

But as he began researching Wynne’s story, he learned that this ordinary man could also tell some extraordinary lies. In the late 1960s, after he had been imprisoned for his spycraft and could no longer assist MI6 nor the CIA, the amateur spy authored a pair of books: The Man From Moscow: The Story of Wynne and Penkovsky and The Man From Odessa , that were riddled with falsehoods.

“[Wynne], bless him, for all his wonderful work, was a menace and a fabricator,” says Nigel West , who has written numerous books on British and American intelligence organizations, including two books specifically about fabricators in the intelligence arena. “He just couldn’t tell the truth. It was pathological with him.”

While its standard for Hollywood films to take liberties with the facts, insert composite characters, devise imagined conversations, and smooth-out timelines to ensure a brisk pace, it’s less common for a based-on-a-true-story movie to have to be more truthful than the source material.

O’Connor makes clear that The Courier is “not a documentary,” even as he explains that he took pains to stick to the facts as much as they could be ascertained—drawing on works such as Jerrold L. Shecter and Peter S. Deriabin’s The Spy Who Saved the World: How a Soviet Colonel Changed the Course of the Cold War and other accounts that could be trusted more than Wynne’s own inventions.

“There’s a fair amount of source material from all different kinds of authors, so by reading everybody—not just Wynne’s books, but other historians, and the official history put out by the American side and the Soviet side — I was able to try and work out what made the most sense and what seemed liked disinformation,” says O’Connor.

Even though Wynne wasn’t exactly a reliable narrator for what he did during his time as a secret agent, the materials he smuggled from behind the Iron Curtain were the real thing. After the initial meeting in December 1960, Penkovsky provided Wynne with film of Soviet military documents and later promised more information if an arrangement with British or American intelligence could be made. Wynne dutifully passed the images to his contacts with British intelligence, who established their legitimacy. Thus began their fruitful relationship, one that involved Wynne hosting Penkovsky in London, who was visiting under the pretense of cultivate new opportunities in the West. On this trip, Penkovsky submitted to hours of interviews with British and American intelligence officials about the Soviet Union’s military and political developments.

“Penkovsky’s dynamism and enthusiasm, his wide-ranging and passionate denunciations of the Soviet system and its leaders illustrated with anecdotes, fascinated and captivated the American and British teams,” write Schecter and Deriabin. “Never before had there been a Soviet spy like him.”

Wynne also enthusiastically embraced his role, enjoying the part of a daring secret agent where he could apply his salesman skills to a higher-stakes game. During their visits, Penkovsky and Wynne would get out on the town, visiting restaurants, nightclubs and shops under the cover of talking business, with each man proudly showing the other around his home country. They made an odd contrast—the short, energetic, and thinly mustachioed Wynne alongside the military bearing of Penkovsky—but there seemed to be genuine affection between the two, and this friendship is a central focus of The Courier .

“These guys were in the foxhole together—they each had a secret that only the other man knew,” says O’Connor . “They were alone in the world with this incredible burden except for the other man.”

But the chummy interactions between the agents and Penkovsky’s prolific, even reckless, acquisition of materials grew increasingly perilous—and finally caught the KGB’s attention. After a meeting in Paris in September 1961, Penkovsky’s next trips were mysteriously cancelled at the last minute. When Wynne visited Moscow in July 1962, his hotel room and luggage were searched, and he was tailed during his travels.

On October 29 of that year, just hours after the Soviets stood down during the Cuban Missile Crisis, Wynne went to Soviet-occupied Budapest with a traveling exhibition of British industrial goods, against the advice of his MI6 handlers. Wynne would later relate that as he walked down the steps of an exhibition pavilion, four men suddenly appeared as a car pulled up and Wynne was pushed inside. He was flown to Moscow, imprisoned, and tried alongside Penkovsky, who it would later be learned had been arrested the week before Wynne entered Hungary.

“They had to go through a show trial, basically, so on the stand Wynne accused MI6 of using him as a dupe—he may have just been saying whatever he could say because he worried they might execute him,” says Jeremy Duns , an author of several spy novels set during the Cold War as well as the history book Codename: Hero: The True Story of Oleg Penkovsky and the Cold War’s Most Dangerous Operation .

For his treason, Penkovsky was sentenced to death and executed by firing squad days after the trial ended (though Wynne would later claim he died of suicide). Wynne, despite claiming ignorance of what materials he was smuggling to the West, was sentenced to eight years in prison. After months of negotiations, the British government was eventually able to arrange a trade of Wynne for the Soviet spy Gordon Lonsdale, who’d been arrested the year before and was serving a 25-year sentence in England.

In all, Penkovsky had provided Western intelligence with about 140 hours of interviews and 111 exposed rolls of film, contributing to some 10,000 pages of intelligence reports. The operation was “the most productive classic clandestine operation ever conducted by the CIA or MI6 against the Soviet target,” as Schecter and Deriabin put it, and key to its success was the mustachioed courier with no prior intelligence experience.

“Penkovsky gave a huge amount of details about what missiles the Soviets had, how old they were, how there were queues for food—it was an extremely vivid portrait of the country and the people within intelligence,” says Duns. “He was senior enough that you could sit down with the agents for hours and explain the entire context of how Soviet intelligence worked.”

Among the materials Penkovsky provided to Wynne were four photocopies of plans for construction sites of missile-launching installations in Cuba. This gave American officials a clearer picture of what the Soviets were doing in the region, bringing in medium-range ballistic missiles. It also helped Americans to understand how limited the Soviets’ capabilities actually were in the area, so as tensions grew during the Cuban Missile Crisis, Kennedy “knew how much rope he could give [Soviet Premier Nikita] Khrushchev,” as Duns puts it.

Upon release from prison, Wynne’s old life was in tatters—he’d lost much of his business and the time spent in the Soviet prison seemed to have caused long-term damage. Seeking ways to parlay the notoriety he received, he became what Duns calls a “rent-a-spokesperson for all kinds of espionage stuff,” making appearances in the media about anything related to spycraft, whether or not it was anything he had experience with. This led to the publication of his dubious memoirs. At the time, they were largely accepted at face value and sold well. The BBC produced a TV movie based on them. But over time, intelligence experts and those involved in the case, though reluctant to share sensitive information, cast doubt on much of what Wynne laid out in his books.

Wynne’s fabrications range from small to huge. In one of his biggest whoppers, Wynne explains that he and Penkovsky took a trip together in a private military jet from the U.K. to Washington, D.C. The two then visited the White House where President John F. Kennedy personally thanked them for their service—then the two returned to the U.K. just 18 hours later. Not only was this account widely denied shortly after publication by members of the CIA and Kennedy’s staff, but it would have been against the way espionage is run—keeping heads of state a safe distance from the details of intelligence work. To top it off, it would have been physically impossible at the time.

“In 1961, jet travel did not allow someone to fly from the U.K. to the U.S. and back again in 24 hours,” says West.

Why did Wynne make up so much, when the truths of his 18 months as a spy are already filled with astounding details? Among the explanations are a desire for money or fame, a ruinous case of alcoholism, or perhaps even psychological scars left by his time in Soviet prison or the shame he felt for publicly turning against British intelligence during the trial. West maintains that it’s the result of something all too typical in the intelligence community—what he calls “post-usefulness syndrome.”

“Imagine that I recruit you and I tell you that whatever you report to me, within an hour, it will be on the president’s desk. You, in your own mind, have developed this sense of self-importance,” says West. “Then after your service, when you haven’t even told your family or friends about this, you’re told, ‘thank you very much, indeed. Don’t call us, we’ll call you in a couple years.’ When Greville got out of prison, he was not prepared, as people obviously are not in those circumstances, to be ignored.”

When it came to writing the screenplay, O’Connor laments that the true story of Wynne’s experiences may never be known. Even the official accounts put out by American and Russian authorities regarding the Penkovsky affair include disinformation and spin that he, or any historian, has to navigate through.

When it comes to espionage, it’s hard to know who to trust.