"12 Years a Slave" has some of the awkwardness and inauthenticity of a foreign-made film about the United States. The dialogue of the Washington, D.C., slave traders sounds as if it were written for "Lord of the Rings." White plantation workers speak in standard redneck cliches. And yet the ways in which this film is true are much more important than the ways it’s false.

Indeed, it’s embarrassing for America that a British director, Steve McQueen ("Shame"), should have had to make this film at all, and that in 2013 it should constitute a breakthrough in cinema for American slavery to be depicted as something entirely evil. Hollywood movies may have come a long way since the days of Uncle Remus and of Mammy in "Gone With the Wind," but the tacit gentlemen’s agreement not to press the issue — not to go too far in rubbing the South’s face in it — has kept Hollywood reticent up to the present, much more reticent, say, than Germany has been about its Nazi past.

"12 Years a Slave" is anything but reticent. It is brutal, at times too brutal, and though the title of the movie makes you aware the horror ultimately ends for one man, the viewer cannot for a minute think of that as a happy ending. We are simply too aware — we are made aware, and in ways that we can never forget — of the suffering that never ended, of the abuses never redressed, and of the anonymous lives that were rendered hopeless for generation upon generation.

McQueen is particularly good at penetrating the specific perniciousness of Southern slavery. In pagan times, for example, if you became a slave, it was your bad luck — no slave holder needed to pretend his slaves were less than human to justify keeping them. But in the American South’s gruesome costume drama, nouveaux aristocrats aped the appearance of European royalty and pretended to themselves and the world that they were Christians. The result of this pretense was a perversion of self that bred sadism and decadence.

McQueen descends into this maelstrom to tell the true story of Solomon Northup (Chiwetel Ejiofor), a free black man who was drugged, kidnapped and sold into slavery. And so, with the sensibility of a modern and refined man, Solomon enters the misery already in progress — the rapes, the beatings, the whippings, the murders, the vicious-imbecile overseers, and the hoop-skirted belles who watch from the porch. He has one master (Benedict Cumberbatch) who, in another culture, might have been a kind man, but now is merely weak. Then he has another, played by Michael Fassbender, in a portrait of a man so sexually and ideologically twisted, and so pleased with himself, that he calls to mind Ralph Fiennes’ death-camp commandant in "Schindler’s List."

Such a chamber of horrors would be better suited to the sensibility of an Edgar Allan Poe than most Hollywood filmmakers. You have to have a penchant for that kind of descent, which is where "12 Years a Slave" becomes, at times, frustrating and contradictory. When this movie violates its own case and crosses into the Grand Guignol — for example, the close-ups of the skin flying off a woman’s back as she’s whipped — the excess feels much more egregious than if a bad movie did the same. It’s not that the subject doesn’t require it but that the violence becomes voluptuous, not Spartan, as though the filmmaker fell in love with his film’s perverse aesthetic.

Yet these moments are more than balanced by scenes of genuine originality, horror and truth, as when we see images of naked slaves on display in a genteel parlor, or when we most unforgettably become witnesses to the nighttime entertainments of the mad slave owner (Fassbender), who insists that his slaves dance for him. In such moments, Fassbender — and McQueen — capture the specific flavor of a complex psychopath who has just enough feeling left to drive him to further elaborations of creative violence.

When "12 Years a Slave" suggests the depths of such spiritual and pragmatic cruelty, it provokes recognition in us of the very heart of darkness. It makes the unimaginable imaginable.

Sarah Paulson, as the slaveholder’s wife, makes clear an evil woman’s psychological sleight of hand: She hides her wickedness to herself by imagining herself injured by her husband’s lust for a young slave. As Patsey, the poor young woman who has the misfortune of being her sick master’s favorite, Lupita Nyong’o embodies with unforgettable force the agonies of body and soul.

Brad Pitt, who is also one of the film’s producers, shows up in a small but crucial role that reminds us that he is the worst and best actor in movies. His accent is weird, and you don’t believe him in the role for a second, and yet when he’s onscreen, it’s great to see him, and there’s no desire to look at anyone else.

Finally, there’s Ejiofor, who is magnificent, playing a man trying to maintain his dignity while hiding his intelligence. His face is the audience’s locus of meaning — the way he looks at the monsters surrounding him is the way we feel. He is never better than in a long close-up, at a funeral, in which Solomon is revived in his will to live by joining in a Negro spiritual. There he stands, in an upside-down world in which villains go unpunished. He has friends who disappear — he never knows what became of them, and so we never know — their torment lost in the darkness of time, along with those of millions.

For Solomon and for McQueen, the music is all that’s left of them, the collective soul that can’t be killed.