The world will never see Mark Felt's body language as he told Bob Woodward about the Nixon administration's illegal behavior or hear the timbre of Daniel Ellsberg's voice when he handed over the Pentagon Papers. But thanks to Citizenfour, a new documentary film by Laura Poitras, there is a digital record of the Hong Kong encounter between NSA whistleblower Edward Snowden and the journalists to whom he revealed mass surveillance of innocents, as well as verbatim excerpts from the encrypted notes he used to facilitate the meeting. For that reason alone, the film will endure as an important historical artifact.

Citizenfour's broader subject is the surveillance state that metastasized in the U.S. and partner countries in the years after the September 11 terrorist attacks. While the film is less thorough and detailed in explaining how the U.S. government is spying on its citizens than Frontline's vital two-part documentary, Poitras's spare portrayal of the global surveillance state is dramatic, accessible to the lay viewer, and accurate—a difficult trifecta given how complicated is the subject matter. Her terse style is exemplified by her treatment of official mendacity. There are scores of instances of U.S. officials misleading or outright lying to the public about surveillance. They are boiled down in the film to two perfectly chosen clips. Then-NSA chief Keith Alexander and Director of National Intelligence James Clapper utter lies so direct, blatant, and egregious even a newbie to the subject can't miss them.

For those who've carefully followed the Snowden leaks and their aftermath in great detail, Poitras's skill handling well-trod subjects in a minimalist but accurate manner inspires confidence in the editing she's done to exclusive scenes of Snowden when he was holed up in Hong Kong hotels leaking information as she recorded.

By necessity, the audience sees only a tiny subset of those moments.

Prior to the film's world premiere Friday at the New York Film Festival, the most detailed account of how the leak unfolded had come from journalist Glenn Greenwald, who portrayed his source as a self-sacrificing idealist with a humble demeanor and an aversion to the spotlight. A counter-narrative advanced by various government officials and opinion journalists has sought to portray Snowden as an arrogant, fame-hungry narcissist who may very well be a spy for Russia or China.

The footage made newly public in this documentary is consistent with Greenwald's portrayal, and undercuts some claims of Snowden's more speculative critics. But due to space constraints, a prudent decision to treat government surveillance rather than Snowden as the film's subject, and perhaps a journalistic desire to protect her source, parts of Snowden's journey from Hawaii to Hong Kong and eventually to asylum in Russia are vague or absent.

But by the film's end, it becomes clear that Snowden's substantial presence in foregoing scenes serves not only to tell the story of the documents he leaked and to slake our natural curiosity about the character of the whistleblower, but also to complete a previously hidden narrative arc. In a final-act reveal, the documentary shows how Snowden's actions—and perhaps his words about them—inspired another national-security whistleblower to contact Poitras and Greenwald via encrypted channels. That anonymous leaker was the source of The Intercept's document-based reporting on America's expansive watch-list of suspected terrorists. Whether more stories will follow from that source is left ambiguous.

Should Citizenfour do well in its theatrical release on October 24, when it will open in New York, L.A., San Francisco, and Washington, D.C., one wonders if it will inspire still more whistleblowers. The opportunity for cultural influence is there. There is Oscar talk around the project, the Los Angeles Times reports, "with most observers feeling it was an instant front-runner to win best documentary and some even speculating about whether it could make a run in other categories. That attention should help galvanize its run in theaters, where few documentaries have much commercial traction. HBO Documentaries boarded the project more recently and aims for a television airing in the spring of 2015."

Would-be whistleblowers will be getting a mixed message. The film conveys the anxiety-inducing uncertainty that Snowden—and those working with him—accepted as a result of their efforts to expose suppressed truths, as when Snowden nervously laments the effect his actions might have on his family or when Poitras and Greenwald converse about whether it's safe for them to go home. All seem aware that the worst-case scenario could destroy their lives.

But the film also captures the deep satisfaction that their work gave them. "I feel good in my human experience to know that I can contribute to the good of others," Snowden says in one scene. In another, Greenwald delights in a righteous "fuck you" to a spying government he regards as criminal and officials he sees as betraying core liberal principles. At film's end, we learn that Snowden is living with his longtime girlfriend in Moscow in circumstances far more pleasant than a Supermax prison, though how long he'll be permitted to remain there is anyone's guess.

History is rife with dissidents who took satisfaction in various causes—some worthy, others abominable. Snowden's critics will continue to insist that his actions were unjustified, no matter how earnest he appears to be about the nobility of his purpose. Yet I suspect that even they will find some merit in this film, if only for its footage. Seldom has the public gotten so intimate a glimpse at how a key figure felt and acted in private moments of profound historic consequence.

Though Poitras has a personal stake in the story she's telling, antipathy toward the surveillance state, and evident affection for what is surely the best source of her career, Citizenfour gives every impression of bringing a documentarian's ethic to the story it tells. In this regard, it compares favorably to the propagandistic stories the U.S. government has told in recent years after events as varied as the rescue of Jessica Lynch, the death of Pat Tillman, the killing of Osama bin Laden, and the Snowden leaks themselves. Additional facts about those events repeatedly contradicted the first accounts fed by officials to a hungry, credulous press.

If Poitras releases hours of raw footage from Hong Kong into the public record, as she told a Los Angeles Times interviewer that she may ultimately do, one gets the impression that her film will prove true to her source material. Meanwhile, mass audiences will soon get an edited look at Snowden that's at odds with much government propaganda about him, with effects on public opinion that remain to be seen.

"You ask why I picked you," Snowden wrote Poitras early in their correspondence. "I didn’t. You did. The surveillance you’ve experienced means you’ve been selected, a term which will mean more to you as you learn about how the modern SIGINT system works."

He went on, "From now, know that every border you cross, every purchase you make, every call you dial, every cell-phone tower you pass, friend you keep, article you write, site you visit, subject line you type, and packet you route, is in the hands of a system whose reach is unlimited but whose safeguards are not. Your victimization by the NSA system means that you are well aware of the threat that unrestricted, secret abilities pose for democracies. This is a story that few but you can tell."

I suspect most Americans who can imagine themselves being treated as the director has by their own government will leave the theater thankful for her film.