Laura Poitras, Dirk Wilutzky, Glenn Greenwald, Lindsay Mills, and Mathilde Bonnefoy onstage during the Academy Awards. Photograph by Kevin Winter/Getty Images

"Thank you to Edward Snowden for his courage," Laura Poitras, the director of “Citizenfour," said as she accepted the Oscar for best documentary. Neil Patrick Harris, the award show's host, noted that Snowden couldn't be there "for some treason." Treason isn't one of the crimes Snowden has been charged with—the government wants to prosecute him under the Espionage Act—but both the praise and the joke point to why this Snowden Oscar mattered. What he did was useful, and dangerous.

That wouldn't have been enough if the movie were bad. But “Citizenfour” is worth watching, as well as celebrating. One still has to ask where the cinematic romance is. At the Oscars, an answer was provided by the young woman onstage with Poitras: Lindsay Mills, the woman whom Snowden at first left behind when he left his job and everything else for a hotel room in Hong Kong. One of the minor revelations of "Citizenfour" was that Mills had joined him in Moscow.

“Just walk me through it,” Glenn Greenwald tells Edward Snowden, in that Hong Kong hotel room. The guidance Greenwald and his colleagues look for is of three distinct kinds: How do you keep secrets? Why would Snowden tell secrets? And what has the government been hiding?

The first is the most one-sided. Greenwald, as the narration delicately makes clear, initially can't figure out or can't be bothered to set up the encrypted line of communication needed to satisfy the mysterious source who e-mails him—this is why Snowden turns to Laura Poitras, who knows exactly what he's talking about when he asks, in their first exchanges, about her public keys. (George Packer wrote a Profile of Poitras for The New Yorker.) Snowden shows Greenwald how to do it (“It seems hard, but it's not—this is super-easy”), and why he should. Here is one of the practical, paradoxical gifts of the Snowden affair: don’t give up on the idea that your words can be secret, at least slightly more secret than is convenient for companies or spies. If you are a little disciplined, you can be freer. There is a lovely shot of Greenwald’s face when Snowden, who is about to enter a password, asks for his “magic mantle of power,” a red sweatshirt, and pulls it over his head, as if he were a man running in the rain, or a teen-ager with a flashlight under his blankets. Looking at him, Greenwald, whom we’ve already encountered as a big talker, is, for a moment, only quiet and curious, with barely a flicker in his expression before he asks, “Is that about the possibility of—overhead?” Greenwald adds that nothing will surprise him anymore. His tone in that instant is one that the film, for all the scenes with angry activists, ultimately shares, and why the film works—neither titillated nor portentous, and just abashed enough to keep its importance from becoming self-importance.

Narcissism is the charge that’s thrown at Snowden—that he thinks he gets to decide what’s secret. His character, or, rather, his motivation for leaking, is the second puzzle for Greenwald and for Ewen MacAskill, the Guardian reporter also in the hotel room. Here, it is MacAskill’s face that is revealing. Greenwald seems sure of what category to put Snowden in, once he is persuaded that the leak is for real and the information is good. (“The fearlessness and the ‘fuck you’ to, like, the bullying tactics has got to be completely pervading everything we do.”) MacAskill, though, begins by telling Snowden that he doesn’t know anything about him; when Snowden starts talking about the N.S.A.’s relation to Booz Allen Hamilton, his on-paper employer, MacAskill stops him: “So, I don’t know your name.” He takes notes; his glances, when he looks up from writing in the scenes that follow, suggest a skeptic’s trust being earned.

The journalists’ relationship to Snowden opens up other sets of questions: What obligations do reporters have to their sources? The legal jeopardy in which James Risen, of the Times, found himself when the government moved to prosecute Jeffrey Sterling, a C.I.A. employee who it argued was Risen’s source, outraged journalists. And rightly so: the Obama Administration, in going after leakers, has pioneered the use of inappropriate legal instruments like the Espionage Act of 1917, which was also used to charge Snowden. (Ben Wizner, of the A.C.L.U., offers a good primer on the Espionage Act in a scene in “Citizenfour,” in which a group of lawyers meet in Berlin; Wizner also says that their phones should go in the refrigerator while they talk, which is funny but not a joke, the opposite of Neil Patrick Harris's "some treason" line). The government ultimately decided not to jail Risen, but Sterling was convicted last month, and will be sentenced in April. Last week, after Attorney General Eric Holder bragged about the Administration’s press-freedom legacy, Risen went on what was variously called an “epic” or “vitriolic” “Twitter rant.” (“I plan to spend the rest of my life fighting to undo damage done to press freedom in the United States by Barack Obama and Eric Holder.”) Margaret Sullivan, the Times’s public editor, wrote that it was actually a pretty reasonable statement of journalistic principles: “Maybe the tenor of Mr. Risen’s tweets wasn’t very Timesian. But the insistence on truth-telling and challenging the powerful is exactly what the Times ought to stand for. Always.” The sense that intemperance can be publicly useful is one of Snowden’s legacies, and Poitras’s film captures it.

In “Citizenfour,” Snowden has a lot to say for the notebook, blog, and camera. He is pronouncing sentences that he seems to have rehearsed in his mind for months or years. He is a guide in the third area that needs to be charted: What is in these documents, and what does all their technical language mean? The danger here, given the number of documents, is that the film could become one big PowerPoint presentation—“just walk me through.” Mostly, Poitras avoids that, once the film gets past the first fifteen minutes. She sketches some programs, offers a route into others, and adds in long shots of things that are not only on paper or in code: an N.S.A. building under construction in Utah, the spot on the British coast where cables disappear under water. There is enough here to make a person better-prepared, later this year, when the authorization for the N.S.A.’s phone-metadata program is set to expire. President Obama’s comment on the matter—“No, I don’t think Mr. Snowden was a patriot”—feels badly off base. The President sounds petty when he insists that he was on top of this without any help from Snowden.

What the country still has to work out is whether the Snowden documents were simply revealing or actually transformative. That’s the question about a good movie, too, though one shouldn’t underestimate the value of revelation, or truth, alone. Snowden has his silent moments. There is a scene, when he is getting ready to sneak out of the hotel in Hong Kong, after he has revealed himself, in which he stands in front of a mirror. Wearing a black shirt, he has put in contact lenses, shaved (after debating the amount of stubble that will make him look least like the pictures now playing on television), and, with a handful of foam, tries to slick back his hair. Watching it again on Oscar weekend, one thinks of Poitras and her team, and all the other filmmakers and actors, getting ready to step out. Snowden tries, and expects, to look different. When he sees that he doesn’t—his hair won’t stay down—he crumples a little, and looks as scared as anyone. There is no magic mantle of power. But outside the hotel room, things really did change.

The New Yorker’s complete Oscars coverage.