RIO DE JANEIRO — A young American lawyer comes to Brazil in 2005, falls in love, finds that his gay relationship confers greater legal rights than back home, starts a blog called Unclaimed Territory focusing on illegal warrantless eavesdropping by the U.S. National Security Agency, takes a place in the hills of Rio with a bunch of rescue dogs, denounces the cozy compromises of “establishment journalists,” gets hired to write a column by Britain’s Guardian newspaper, is sought out by the N.S.A. whistleblower Edward J. Snowden, becomes the main chronicler of Snowden’s revelations of global American surveillance, is lionized for work that prompts a far-reaching debate on security and freedom, files repeated thunderbolts from his leafy Brazilian perch, and ends up in just eight years as perhaps the most famous journalist of his generation.

These things happen. At least they happen in the empowering digital age, and they happen to Glenn Greenwald.

With his gray shirt, black backpack, regular features and medium build, he merges into the Rio crowd, the ordinary man. Over a Thai lunch, he tells me he is sleeping five hours a night, running on adrenalin. So what does he do to relax? “Roll around in the mud with my 10 dogs.”

Unwinding is hard. The five months since he met Snowdenin Hong Kong have been relentless; they talk almost every day. He lives in limbo. “I feel like if I went back to the United States there is a more than trivial chance I would be arrested,” he says. “Not one of 20 lawyers I have spoken to has said, ‘Oh, you are being paranoid, of course they would never think of arresting you.”’

Would Greenwald enjoy First Amendment protection after publishing top-secret information? The record of the Obama administration is ominous. He says his lawyers are unable to get clarification. His mother in Florida asks: “What if I am on my deathbed and cannot see you?”

Greenwald lives with a sense of exile but is pesky in his determination not to relent. He has been embraced as a hero by Brazil after revealing U.S. spying on President Dilma Rousseff (who postponed a planned state visit to Washington) but has resisted one request to hand over documents and is determined, here as elsewhere, to keep his distance from power.

He is on a double mission: to push back in the name of freedom against the post-9/11 “surveillance state” with its dragnet data trawling, and to reinvigorate journalism through “an aggressive and adversarial position to political and corporate power,” an undertaking he will pursue through a new online publication backed with $250 million from the eBay billionaire Pierre Omidyar, the same amount Jeff Bezos of Amazon paid for The Washington Post (a sobering reflection on the standing of legacy newspapers today.)

On the first of these fronts, he says he is only halfway through the reporting of Snowden’s documents “with a lot of huge revelations to come.” On the second, explored in a recent exchange with my colleague Bill Keller that will be taught in journalism schools, he has already made about 10 hires. (He and Omidyar have never met, which must be some sort of first for such a venture.)

“Our style will be to encourage and empower combative journalism that can be a real force against powerful people,” he says. “We want our journalists to follow their passion.”

He continued: “The reason why journalism is important, why it is protected in the Constitution, is to be one of the institutional checks on abuse of power, and for that you have to keep those in power at arm’s length, hold them accountable.”

For Greenwald, American journalism has been defanged by the “patriotism compulsion” after 9/11 and by the culture of big media corporations. He alludes to David Halberstam’s speech at Columbia University in 2005: “Never, never, never, let them intimidate you. People are always going to try in all kinds of ways. Sheriffs, generals, presidents of universities, presidents of countries, secretaries of defense. Don’t let them.”

Of course, this admonition is sacred to plenty of old-school journalists. Greenwald overstates the conformity of mainstream papers, whose investigative journalism is often vigorous and fearless. But he is right that journalism got engulfed, with grave consequences, in America’s great post-9/11 disorientation. And there is no question journalism will benefit from having the personal, open-with-its-bias reporting Greenwald proposes alongside the impartiality-seeking traditional media. “Biased and balanced” — the Andrew Sullivan blog formula — is an important component of the new media landscape. Each form can spur the other, keep it honest.

American society will also benefit from Greenwald’s ongoing revelations about out-of-control surveillance. He has testified before the Brazilian Senate, and should be allowed to testify before the U.S. Senate. He says, “I am definitely going back, I refuse to be exiled for a lie.”

He deserves assurance that he can return to the United States without facing arrest.