Hermitage, of course, had its struggles. In 1998, Russia defaulted on its foreign debt. The ruble crashed. The company’s assets plummeted to $100 million. But over the next seven years the price of oil quadrupled. And Browder, determined to ride it out, made a killing, thanks to his major stake in Gazprom, Russia’s state-owned oil-and-gas colossus. By the early 2000s, he had decided on a brash move. He started a campaign to expose the endemic corruption at various Russian firms. He publicly charged an array of businessmen and officials, accusing them of looting their companies’ assets. Soon, alarmed that the oligarchs might try to assassinate him, Browder enlisted a security detail and began traveling in a three-car motorcade.

As Browder’s business thrived, so did his hubris. Despite his attempts to curb corporate graft, Browder spoke out favorably about Putin, telling The New York Times that he “has turned out to be my biggest ally in Russia.” Browder’s rationale for supporting him? “We want an authoritarian—one who is exercising authority over the Mafia and oligarchs.” During our many conversations, Browder would try to put these statements in context: Putin had promised “that he was going to restore order and stamp out the abuse by the oligarchs. Everyone was hoping that he was going to be true to that pitch.” Browder, meanwhile, said he was simply “trying to make all my money back.”

Earl and Raisa Browder became pariahs. In their home, they hung portraits of Stalin, Lenin, and Mao.

He would soon realize, though, that he was being played and that his status as a foreigner did not insulate him from the authorities’ wrath. In November 2005, after deplaning at Moscow’s main airport, he was detained by immigration officers, held overnight, and deported. He had become yet another wealthy businessman who had suddenly fallen out of Putin’s favor. As Browder recalled, he was literally and figuratively “the last person waiting for the baggage on the airline carousel.”

At our first session, in London, Bill Browder did not act like someone who feared for his life. I had expected to find elaborate security at his building. Instead, I was given a perfunctory glance and waved up to the Hermitage offices, which overlook London’s picturesque Finsbury Square. I began to wonder if there was a hidden security system in place, speculating, as others have, that Browder either was protected by or had a special relationship with M.I.6, Britain’s foreign-intelligence service. Curious, I asked him, point-blank, “Are you M.I.6?” He hesitated and then said, cryptically, “I work with so many levels of British law enforcement, I never know who is working for whom.” When I broached the subject again, months later, he bristled: “I am definitely not under British protection. I have to protect myself. I am not M.I.6. I am not C.I.A. I am not anything. I bet there are nine intelligence agencies listening as we speak. But that is because I am not an agent of anyone’s—I am a completely independent operator. If I were really an agent of M.I.6, why would I be having so many problems opening a criminal case [in the U.K. against the oligarchs]? Maybe this is a double fake-out! The Russians are accusing me of being a C.I.A. agent!”

With the passage of new laws in England, some oligarchs have felt the squeeze. Last May, Parliament—after the poisoning, in Salisbury, of former Russia spy Sergei Skripal and his daughter, Yulia, passed its own version of the Magnitsky Act. And yet the government appears ambivalent about investigating the $30 million in “illicit funds” that Browder contends have been laundered through British banks. “I have gone to every agency,” he said. “No one will take it on. It all stops with the higher-ups. It’s either massive incompetence or collusion.”

This fall, underlying tensions between Browder and the Brits came into stark relief when detective Jon Benton, the onetime head of the international-corruption unit looking into these suspicious dealings, made a surprising admission. He said he had received orders to stop his inquiry—from a more senior official with links to the Foreign Office. “I was approached and taken for a coffee,” Benton told The Sunday Telegraph, “and basically told Bill Browder is a pain in the neck and to leave it.” Browder was livid but not surprised: certain members of the government have reason to keep in with the Kremlin; some of the Russian business elite retain strong connections with England’s power players. Earlier this year, according to The New York Times, billionaire Oleg Deripaska, a Russian aluminum magnate suspected of having connections to organized crime, retained the lobbying help of a British lord, Gregory Barker—an associate of David Cameron—hoping to sway U.S. officials to lighten or remove stiff sanctions on Deripaska and his businesses. “The U.S. won’t respond to this kind of pressure,” Browder told me in November. “They’d look like they’re caving.” But back in England, Browder said, fuming, “We are still trying to get the U.K. to make a list of sanctioned individuals. They are stalling.”

RUSSIA’S TOP COMRADE

Bill’s grandfather Earl Browder in the late 1930s; at the time he served as the leader of the American Communist Party. Photograph from Universal History Archive/UIG/Getty Images.

Browder’s office resembles a small trading floor, with analysts sitting in rows of desks, staring intently at computer monitors. Many spend their time rifling through complex chains of transactions in far-flung foreign bank accounts to determine which ones can be traced to corrupt culprits. So far, Browder has sought to help two dozen nations recover their piece of the $230 million pie—the tax money paid to the Russian Treasury—that he alleges to have been stolen by Russian officials, moguls, and mobsters. A handful of countries, including Switzerland, France, and the Netherlands, have opened investigations. In September, the head of Denmark’s Danske Bank abruptly resigned after authorities, with an assist from Browder, opened a $234 billion money-laundering probe of its Estonian branch.