Hervé Falciani, who took client data from H.S.B.C., was indicted in Switzerland, jailed in Spain, and celebrated in France. Photograph by Moises Saman / Magnum for The New Yorker

A few days before Christmas in 2008, Hervé Falciani was in a meeting at his office, in Geneva, when a team of police officers arrived to arrest him. Falciani, who was thirty-six, worked for H.S.B.C., then the largest bank in the world. He was on the staff of the company’s private Swiss bank, which serves clients who are wealthy enough to afford the minimum deposit—half a million dollars—required to open an account. Falciani had been at H.S.B.C. for eight years, initially in Monaco and then in Geneva. He was a computer technician who helped supervise security systems for the handling of client data. He had grown up in Monaco, where as a young man he had worked as a croupier at the Casino de Monte-Carlo, and developed an excellent poker face. As the Swiss police escorted him from the building, he insisted that he had done nothing wrong.

Officers questioned Falciani at a nearby station. They were investigating a data theft from the bank. Since 1713, when the Great Council of Geneva banned banks from revealing the private information of their customers, Switzerland had thrived on its reputation as a stronghold of financial secrecy. International élites could place their fortunes beyond the reach of tax authorities in their own countries. For Swiss wealth managers, who oversaw more than two trillion dollars in international deposits, the promise to maintain financial privacy was akin to a religious vow of silence. Switzerland is the home of the numbered account: customers often specify that they prefer not to receive statements, in order to avoid a paper trail. In light of these safeguards, the notion of a breach at H.S.B.C. was shocking.

Police officials told Falciani that someone calling himself Ruben al-Chidiak had stolen client data from the bank. They weren’t sure how much information had been taken or how the theft had been engineered. But they suspected that Chidiak was a pseudonym, and that the real culprit was Falciani.

Falciani told the police that his job was to protect data: How could they accuse him of compromising such information? As darkness fell, he asked to go home. His wife, Simona, would be worried about him. The investigators released him, but instructed him to return for further questioning the next morning.

Falciani walked through streets strung with Christmas lights to his apartment, in a dingy building on the Rue des Mouettes. He and Simona packed a few bags, bundled their three-year-old daughter, Kim, against the cold, and prepared to flee the country. Despite his protests, Falciani had stolen the data.

When the Falcianis walked out of their apartment, they left the keys in the door. Falciani rented a car, and they drove through the Alps. The next morning, as Swiss investigators assembled at the police station in Geneva, Falciani was approaching the South of France. He left the rental car at the airport in Nice. His wife and daughter went on to Italy, for a visit with Simona’s family; Falciani travelled to his parents’ home, in Castellar, a hill town near the French-Italian border.

W. Somerset Maugham once described the Côte d’Azur as “a sunny place for shady people,” and Falciani, who was now a fugitive, hunkered down in Castellar. As a precaution, he had not travelled with the stolen data, instead uploading the information to remote servers. He now downloaded the files onto his laptop. The Swiss had asked French authorities to help track down Falciani, and at dawn on January 7, 2009, gendarmes raided his parents’ house. The prosecutor in Nice who handled the case, Éric de Montgolfier, told me that authorities in Switzerland were so eager to seize Falciani’s computer that they sent a Swiss prosecutor to accompany the gendarmes.

The French police arrested Falciani and seized his MacBook Pro and his iPhone. But when he was out of earshot of the Swiss prosecutor, on the way to the police station in nearby Menton, he told the gendarmes that his computer contained information of possible interest to the French state: names, account numbers, account balances. The hard drive held evidence, he said, of “tax evasion committed by French people.” Falciani had obtained sixty thousand files relating to tens of thousands of H.S.B.C. clients from nearly every country. An H.S.B.C. lawyer later described Falciani’s crime as “the largest robbery of a bank ever committed in the world.”

Falciani’s flight to France coincided with the onset of the global financial crisis. Many countries were scrambling to secure revenues and crack down on citizens whose fortunes were stashed in offshore tax havens. Years before the leak, this April, of the Panama Papers—a cache of documents from Mossack Fonseca, a law firm in Panama City that specializes in the creation of anonymous shell companies—there was ample evidence that the global plutocracy has many outlets for dissimulation in the realm of personal finance. “Only the little people pay taxes,” the billionaire Leona Helmsley once remarked—to her housekeeper. In 1989, the housekeeper recounted the exchange to a New York jury, and Helmsley spent eighteen months in prison. Most tax evasion, however, goes unpunished.

According to a 2012 study by James Henry, a former chief economist at McKinsey who now advises the Tax Justice Network, the world’s wealthiest people salt away at least twenty-one trillion dollars beyond the reach of tax authorities. In a book published last year, “The Hidden Wealth of Nations,” the economist Gabriel Zucman offers a lower, yet still enormous, estimate: $7.6 trillion, or eight per cent of the world’s personal financial wealth. Zucman calculates that “the fraud perpetuated through unreported foreign accounts each year costs about $200 billion to governments throughout the world.”

The data that Falciani stole could function as a treasure map, enabling a country like France to recover some of that lost revenue. Montgolfier said, “When you have so many French people with Swiss accounts”—he raised his eyebrows and his shoulders in a synchronized Gallic shrug—“it has a perfume of fraud.”

The Swiss prosecutor demanded that Montgolfier turn over Falciani’s laptop, but he demurred. “We’ll look at the computer,” he said. “Then we’ll decide if we return it.” To the Swiss government, Falciani was merely a thief, but the French saw him differently. “I would characterize him as a bit messianic,” Montgolfier told me. “There was the context of the world crisis, provoked by finance and all these big banks enabling tax evasion, and this guy just wanted to set the world free of those behaviors.” In a memoir recently published in Europe, “Earthquake on Planet Finance,” Falciani writes of his motives: “I wanted a different world for my daughter. I didn’t want her to grow up in a reality where money rules, where the abuse of power and the constant bypassing of the rules was the norm.”

As if to underline the incendiary implications of Falciani’s data, Montgolfier placed the laptop in a safe. While French authorities deliberated how to proceed, Falciani spent the night in a holding cell in Menton. But the next morning, in a gesture that indicated a shift in Falciani’s status, his guards surprised him with coffee and croissants.

When I first met Falciani, on a winter day at the Place d’Italie, in Paris, in 2014, he had been living under police protection, fearful that his life was endangered because of the information he had exposed about unscrupulous élites. He often travelled with three bodyguards, who were provided by the French state, but when we met Falciani arrived alone, on a fold-up scooter. He had proposed a curious venue for our meeting: Hippopotamus, a chain restaurant that caters to French children, with a cartoon mascot and colorful menus featuring an array of tiny steak frites.

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Falciani ordered a slice of cheesecake. He was dressed in the manner of a Tarantino assassin: white shirt, skinny black tie, aggressively tailored black suit. He is soap-star handsome, with a dimpled chin, olive skin, and what one French newspaper described as “a commercial smile.” His sideburns tapered to a sliver. “My father worked in a bank,” Falciani said, in accented English. As a child in Monaco, which is one of Europe’s oldest tax havens, he often accompanied his father to work and marvelled at the discreet power of the institution. The bank was immaculate, and everybody spoke in hushed tones. It reminded Falciani of a church. After business hours, he liked to dash through the carpeted hallways.