After World War II, a critical MI5 spy named Juan Pujol Garcia faked his own death, and kept it secret for almost four decades. And that’s not even the most interesting thing about him.

Pujol was a veteran of the Spanish Civil War who had come to loathe totalitarianism, both in Francisco Franco and Adolf Hitler. When Britain went to war with Germany in 1939, he was determined to join the British war effort as a spy against Germany; so determined that he wasn’t deterred when British officers turned him down because he didn’t have any connections or credentials. Instead, he formulated a plan to pad his resume while aiding the war effort.

Posing as a Spanish official who was flying to London, Pujol made contact with Nazi officials in Madrid and told them that he was interested in spying on Britain for the Third Reich. After that, he began sending the Nazis fabricated information that they thought was from London, but which was actually fed from Lisbon and Madrid. Essentially, Pujol became a rogue double-agent whom Britain didn’t even know it had.

Though Pujol sent Germany false reports, he used lots of factual information to make them seem legitimate, says Stephan Talty, author of Agent Garbo: The Brilliant, Eccentric Secret Agent Who Tricked Hitler and Saved D-Day and a forthcoming book about a Cold War rescue mission.

“He was really gathering these sort of factoids from different encyclopedias, and even from advertisements he saw and placards he saw in the street,” Talty says. “So he was a complete amateur, but he was able to sort of built up enough of a portfolio to finally approach the British.”

That isn’t to say he didn’t make some mistakes. Britain’s domestic intelligence agency, MI5, notes on its website that Pujol once told Germans that on a visit to Glasgow, Scotland, he found men who “would do anything for a litre of wine.” Evidently, the Nazis didn’t catch that he should’ve said “beer” or “whiskey” (in 2015, the Telegraph declared “Scotland’s first wine branded ‘undrinkable’ by critics”).

In 1942, Pujol again approached British officials about becoming a double-agent by showing them that, in fact, he already was one. Unbeknownst to him, British operatives had already realized that a secret spy was sending information to Germany from Portugal and Spain, but didn’t know who that spy was. When Pujol revealed himself, they brought him to London to work for MI5.

The Nazis continued to think of Pujol as an important spy throughout the war. They never discovered that he was a double agent, despite the fact that a lot of his information was incorrect.

“I think the Germans felt that no one could really fake this much information and this many different characters,” Talty says, referencing the 27 imaginary spies that Pujol told Germans he had recruited to feed him information (Pujol’s MI5 codename was “Agent Garbo” because he was such a good actor). “[The Germans] also felt that if they cut him off or if they doubted him even, they were not going to just lose one agent, they were going lose a network.”

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In his most famous deception, Pujol told the Nazis that news they’d heard about a planned invasion of Normandy was fake. Of course, this wasn’t true, and as a result the Nazis were unprepared for the Allies’ successful D-Day invasion.

After the European war ended in 1945, Pujol continued to work for MI5 to investigate whether Germany had any plans to resurrect a sort of Fourth Reich. After this, Pujol wanted to get out of Europe and away from his memories of the war, so he moved to Venezuela. But because many former Nazis had also chosen Venezuela as a place to flee from their crimes, Pujol figured it would be safer for him there if everybody thought he was dead.

In 1948, he called Tommy Harris, who’d been his handler at MI5, and instructed him to tell everyone that he had died of malaria in Angola. Harris spread this news through the organization, and a year later the British ambassador officially told Spain that he was dead. The news reached Pujol’s first wife and children in Spain. Meanwhile, in Venezuela, Pujol grew a beard and started wearing a distinctive pair of glasses.

Pujol kept his secret until the 1980s, when the British writer Nigel West looked into his life and theorized that he hadn’t died. West tracked down Pujol, who came out of hiding and announced that it was true. He returned to Europe and reconnected with his ex-wife, who’d always suspected he was alive, and his very-upset children, who had not.

He died a second—and final—time in 1988, but not before writing a book about his life with West (West is actually the pen name of Rupert Allason, a former member of Parliament who’s been sued several times for libel). Last year, Variety reported that Oscar Isaac will star in a film about his life.

Talty notes that Pujol didn’t have to fake his death for as long as he did. He would have probably been safe from Nazi retaliation had he, for instance, revealed himself in the 1960s. Talty argues that Pujol seemed to be ashamed that he was not able to make a successful career for himself in Venezuela after the war, and that this is part of why he stayed in hiding. In addition, Talty says that Pujol was good at assuming different personas—one of the reasons he was such an unusually good spy in the first place.

“If you read espionage history, so many agents are either trapped into becoming double agents or paid for their work,” Talty says. But that wasn’t the case for Pujol. “He was an idealistic double agent, which is very rare. He did it strictly out of idealism, and I think also to realize himself as the great improvisational actor that he knew he was. It gave him a role that he wanted to play.”