Typically, a movie gets just one opening gambit, and most are too scared to take the opportunity. Asghar Farhadi's A Separation gets two, and it makes both of them count: first, we see passports being photocopied, from a perspective inside the copier, one after the other, a different one every time, without anyone ever coming to replace them. It is scarily decontextualised, and it promises that we're about to see a tale in which papers - the grinding wheels of bureaucracy - are going to play a huge role, and that the specific story we're up for is just one of many that could be told.

It's a great foundation for the opening credits, and it's kids' stuff next to the first scene proper: a single shot of a man, Nader (Peyman Maadi), and a woman, Simin (Leila Hatami) sitting in a blank, obviously official room, staring right at the camera. They are there to request a divorce: Simin wants to leave Iran with their 11-year-old daughter and their visas expire in 40 days; Nader refuses to leave while his father suffers from Alzheimer's. Thanks to the arcana of Iranian law, Simin can't leave the country as a married woman if her husband stays behind; and so within just a few minutes we are presented with an insoluble problem in which both players seem equally right and totally incapable of reaching a compromise. The judge - sitting in our position - does not grant their request, and the shot ends with a close up of Simin, glaring as the judge loftily declares that her issue is a small one. She's staring angrily at him, but the brunt of her ire lands right on us, and no filmmaker could do a better judge of posing the central question of the film to come: do we dare think that we can properly judge what's going on? Do we know what is or should be true about these people?

A Separation - a corruption of the much-better Iranian title which translates as The Separation of Nader from Simin - arrives in the States on the back of just about as much praise as one movie could possibly receive, not least of which is that it's the most-rewarded film in the history of the Berlinale festival. The mere fact that it's not accordingly the best film ever made thus ought to be a touch disappointing, but let's be reasonable. It is, certainly, an instant qualifier for the top tier of modern Iranian cinema, a return to the extraordinary output of Iranian directors in the 1990s and early '00s after a half decade of the national cinema suffering under increasingly severe censorship. A Separation itself had a bit of a run-in with the authorities, which wasn't enough to prevent it from being Iran's official submission to the Oscars, in what I might call a blatant fit of hypocrisy if the film didn't deserve it.

The alarmingly convoluted plot starts out simply, with the situation I have just sketched; following the denial of her request, Simin moves back in with her mother, leaving her daughter Termeh (Sarina Farhadi, the director's daughter) with Nader; and we will not see her again for a very long time, during which things go to hell in the most colorful ways. Nader, at loose ends about what to do with his father (Ali-Asghar Shahbazi), hires a temporary caregiver, Razieh (Sareh Bayet), who is pregnant, unbeknownst to her new employer. It's not her only secret either, for she has refrained from sharing the details of her new job with her unemployed husband, Hodjat (Shahab Hosseini), a religious hardliner with a quick temper.

Every reviewer of A Separation will eventually come to the important point of decision: at what point will I stop talking about the story? That point, for me, is now. Suffice it to say that things transpire which put the two couples in opposition to one another, and set them in opposition within one another, and the film turns invisibly into a procedural thriller of sorts about the search to figure out the truth, guided there by an exhausted, disengaged court interrogator (Babak Karimi). The glee with which Farhadi exploits the gulf between what is on-camera and what happens out-of-frame, and his similar use of the crucial break in continuity caused by an edit, so that we in the audience are always more sure of what we think happened and what we think the characters have done, than we know what happened; this extraordinary manipulation of what we perceive to be "real" in cinema is borderline Hitchcock, if it weren't the case that A Separation was so heavily rooted in the European realist aesthetic that makes its generic elements - and there are undoubtedly elements of genre here, the second hour of the film is as nervewracking as any proper action-adventure of 2011 - play almost as parody; we could as easily call the film an anti-thriller as a thriller, and is surely an anti-procedural.

The film is, in fact, so much fun to watch that you could very nearly miss the important fact that it is also as piercing a critique of Iranian society as that country has produced in some time - since Jafar Panahi was last able to make a feature way the heck back in 2006, at least. Mostly, this is done through establishing oppositions: secular, middle-class Nader and Simin are opposed to religious, lower-class Razieh and Hodjat; the husbands are both autocratic in their own ways, their wives obliged to assert themselves indirectly and with cunning rather than force. Termeh is weary and nervous and always looked worn-out; Razieh and Hodjat's much younger girl Somayeh (Kimia Hosseini) is bright-eyed and inquisitive and full of life (some of the best shots of the movie consist of nothing but lingering shots of the child actress, staring intently). Religion, sexism, and classism are all woven into the film so casually that it never feels like a harangue, nor are the character reduced to symbolic agents, as they could so easily be. Part of this is thanks to the outstanding performances from all four principals - five if we include Farhadi, and six if we really stretch to include young Hosseini - with my pick for best in show going to Hatami, who has perhaps the smallest role by screentime, being as she is packed off for most of the first hour, but who grounds the film once she returns with unmatchable moral authority, and whose focused gaze is the most commanding thing about the whole movie; and with her pale skin and flaming red hair, she stands out visually from rest of the cast, focusing our attention on her intense performance even further.

This is devastatingly great filmmaking: brilliantly shot using an extremely limited palette of compositions that are all used with surgical precision, edited so that its esoteric and largely conversational drama is as riveting as the most salacious mystery. If there was any doubt after his naggingly effective About Elly that Farhadi is a top-shelf international filmmaker, A Separation eliminates it; the final shot alone, a marathon-length job that goes on through the whole of the end credits and relies heavily on the slightest, most graceful subtleties on the part of actors, would be enough to get any director a spot in the best of the year heat. Plain and simple, this is a masterpiece: rich in specific detail about a very particular problem in a very exacting place, but redolent with universalism and human feeling.