Citizenfour must have been a maddening documentary to film. Its subject is pervasive global surveillance, an enveloping digital act that spreads without visibility, so its scenes unfold in courtrooms, hearing chambers and hotels. Yet the virtuosity of Laura Poitras, its director and architect, makes its 114 minutes crackle with the nervous energy of revelation.

Poitras, the first journalist contacted by National Security Agency whistleblower Edward Snowden, mirrors her topic. She rarely appears on news programs or chat shows. She is a mysterious character in her own movie, heard more than she is seen.

But surreptitiously, Poitras has been a commander of a stream of disclosures for 16 months that have forced the NSA into a new and infamous era. Citizenfour demonstrates to the public the prowess that those of us who have worked with her on the NSA stories encountered. Her movie, the culmination of a post-9/11 trilogy that spans a dark horizon from Iraq to Guantánamo, is a triumph of journalism and a triumph for journalism.

At its heart, Citizenfour is the story of how Snowden’s disclosures unfolded through Poitras’ eyes, from the first communications Snowden sends Poitras, hinting at what is to come, until Snowden sees himself vindicated through emulation. (The film is named for a pseudonym Snowden used with Poitras.) The time before Poitras meets Snowden is symbolized by a car travelling through a pitch-black tunnel, barely illuminated by the glowing red lights on the ceiling, until sunlight bursts in when she and her colleagues Glenn Greenwald and the Guardian’s Ewen MacAskill arrive in Hong Kong for their fateful encounter.

Edward Snowden, left, with Glenn Greenwald, second from right, David Miranda and Laura Poitras. David Miranda Photograph: David Miranda

The outlines of that story are now familiar. Snowden, an NSA contractor, provides the journalists with voluminous evidence of industrial-scale surveillance covering much of the planet’s communications, and forecloses on a normal life in the process. Poitras trains her camera on contextualizing both aspects of Snowden’s decision.

Accessibly explaining how surveillance works, and why it matters, only gets more challenging the deeper you dig into the NSA trove. At the Guardian, it consumed exhausting months’ worth of background reporting, verification and endless revisions. Poitras, through Snowden, employs minimal jargon about “selectors” (email accounts, IP addresses, phone numbers, etc). One deft way she demonstrates the breadth of NSA’s reach is to film the security researcher and journalist Jacob Applebaum teaching an Occupy Wall Street crowd about life patterns displayed through their their credit cards, transit passes and phone records – the web of metadata that shows our associations and choices which, out of context, can make us look suspicious. Anyone engaging in modern communications has unsuspectingly provided the NSA with valuable information.

Since June 2013, Snowden has been a cipher to the world, often yielding paranoid reactions (Russian spy! Chinese dupe!) from people understandably curious about his motives. It may be too late to change people’s minds about Snowden, at least so soon after his leaks. But the Snowden who Poitras shows – hair tousled, resisting his attempts at styling it – is determined, sincere and human.

While often portrayed as arrogant, especially by self-interested surveillance bureaucrats, Snowden tells Poitras, Greenwald and MacAskill that he wants journalists and not himself to decide what ought to be public. He is possessed with an uncanny calm as he is about to become forever targeted. Yet Snowden’s eyes redden and his shoulders stoop when he grasps the burden he is placing on his family and girlfriend – with whom he is now reunited in Russia, a place in which he never intended to live.

Facebook Twitter Pinterest Trailer for Citizenfour.

The film leaves many questions unanswered, such as Wikileaks’ role in Snowden’s drama – Julian Assange is briefly on camera – and Snowden’s circumstances in the authoritarian Russia that has granted him asylum. But Citizenfour shows Snowden vindicated when the film confirms that Greenwald, Poitras and their investigative partner Jeremy Scahill are working with a new security whistleblower, one apparently inspired by Snowden. While understated, Snowden appears thrilled, even moved.

Given the passions that the NSA disclosures have generated, it’s remarkable how tempered Citizenfour comes across. Reflecting a style Poitras seems to share with Snowden, it’s a quiet movie, its soundtrack a sinister digital throb, packed tight with questions about how we live freely in an unseen dragnet. One of its only boisterous moments comes when Snowden and Greenwald discuss the spirit animating both the reporting and Snowden’s decision to reveal himself. Greenwald describes it as “the fearlessness and the fuck-you”.

That fearlessness attracted Snowden to Poitras, and it shows through her camera.

Citizenfour opens in US cinemas on 24 October.

