But if he is us, then who is them? The officials from the Obama and George W. Bush administrations who have defended the N.S.A. in court, before Congress and on television, promising that the rule of law and the rights of citizens are being respected, even as the bad guys are being chased down and spied upon? Those presidents themselves, who preach liberty even as they expand the prerogatives of the executive branch? The telecommunications executives who collude in the collection of data?

All of the above, but maybe also not quite any of them. Plenty of movies have tried to imagine the contours of state power, but “Citizenfour” stands alone in evoking the modern state as an unseen, ubiquitous presence, an abstraction with enormous coercive resources at its disposal. To some extent, Ms. Poitras and Mr. Greenwald are engaged in a theoretical inquiry, a kind of speculative mapping, of the shape and reach of this mysterious entity. That is not to say that the United States government’s data collection program is not real, but rather that its extent and implications are only beginning to be understood.

Mr. Greenwald, a prolific writer and prodigious talker (in Portuguese, too!), has made his case against secrecy and surveillance in numerous articles, blog posts, books and television appearances. Ms. Poitras, who does not appear on camera in her film and speaks only when reading Mr. Snowden’s emails to her, pursues a slightly different project. She deploys the tools of her trade — spooky music and fluid editing, subtle camera movements and suggestive compositions — to try to coax a specter into view.

It is everywhere and nowhere, the leviathan whose belly is our native atmosphere. Mr. Snowden, unplugging the telephone in his room, hiding under a blanket when typing on his laptop, looking mildly panicked when a fire alarm is tested on his floor, can seem paranoid. He can also seem to be practicing a kind of avant-garde common sense. It’s hard to tell the difference, and thinking about the issues Ms. Poitras raises can induce a kind of epistemological vertigo. What do we know about what is known about us? Who knows it? Can we trust them? These questions are terrifying, and so is “Citizenfour.”

“Citizenfour” is rated R (Under 17 requires accompanying parent or adult guardian). Not because of the nightmarish spectacle of unchecked state power, but because of the swearing it occasionally provokes.