Shame is a serious movie that wants to be taken seriously. And the bulk of this second feature by the English director Steve McQueen succeeds, revealing an episodic portrait of an unhappy sex addict living the good life in Manhattan. Brandon (Michael Fassbender, whom you might shortly get sick of) has a slick office and a slicker apartment, both of which he uses to watch porn, masturbate, and seduce women, though he’s better at paying for them. Everything’s going swimmingly until his work computer is removed for viruses. (Little chink.) Worse, Brandon’s wayward sister Sissy (Carey Mulligan, whom you will never get sick of) drops in for an indefinite stay. (Huge chink.) Now his lifestyle, once as solid as armor, is being eaten away, and he responds like a wounded animal.

Fassbender is fierce as Brandon, who initially comes off as merely an oversexed and sorta-sad dude. He’s not hurting anybody; he’s still doing his job. But Sissy’s a catalyst that unearths something darker, more desperate, and much more frightening. Brandon’s game has been changed, and he can’t handle it.

McQueen, who first gained acclaim for his work as a video artist, co-wrote Shame with Abi Morgan (who also penned the upcoming The Iron Lady), and its graphic subject matter earned the film an almost unheard-of NC-17 rating. The sex isn’t titillating, however—Brandon isn’t exactly a ladies’ man, just a guy who happens to be handsome enough get women to jump into bed with him. The relations are utilitarian rather than erotic, just another one of Brandon’s daily habits.

At least, that is, until the film’s final chapters, which are marred by two missteps that at best will leave you rolling your eyes and at worst have you laughing out loud. Note to McQueen: Maybe you shouldn’t set that enthusiastic threesome to such heavenly strings, even to denote a turning point in a character’s life; thrusting, breast-grabbing, and face-sucking don’t pair well with an angelic chorus, unless it’s for a giggle. And if your anti-hero must have a meltdown, for God’s sake, don’t have him fall to his knees in the rain. Even The Muppets parodied the sadness-in-the-downpour cliché.

It doesn’t help that Fassbender’s character is such a blank slate. When he and a co-worker go on an actual date, he displays all the personality of a wet newspaper; when the two eventually go to bed, she comes off as evenmore desperate than he is.

Sissy supplies the only hint of a troubled past. In fact, it’s Mulligan’s presence that makes Shame so watchable, at least until Brandon begins to unravel. As always, the actress is vulnerable yet magnetic, needy but headstrong. Sissy’s a singer, and during her one performance — a slowed-down, simultaneously lovely and interminable “New York, New York”—Mulligan is Michelle Williams-mesmerizing. And though Sissy is also a playful wreck with no respect for Brandon’s privacy or professional life (she sleeps with his married boss), his reactions to her transgressions, great and small, almost always feel cruel: “You trap me,” he hisses at her. “You’re a parasite.” At one point, she calls him a “fucking weirdo,” and you pretty much have to agree.

Which is, well, a shame. The film’s subject is novel enough, and Fassbender is incendiary when he’s called on to be. But without knowing a little more about the character—enough to make him seem like a real person and not just a single-flaw outline—shaking his story off your shoulders proves a little to easy. Then McQueen goes for a non-ending, which makes Shame only the latest anticipated art-house flick to drown in ambiguity this year. Closure may come off as too convenient, but question marks are seeming increasingly facile, too.