From its opening frames, there's a jagged, jittery uneasiness about Carmel Winters' debut film, Snap. The source of that feeling is deliberately obscure; it's an unsettling movie, but at first, we're not sure why. A woman, Sandra (Aisling O'Sullivan), is being interviewed by a small documentary crew ("This is mental," she notes, early on), talking about a horrifying event that she has been at least partially blamed for. But it's not the kind of smooth, neat footage that clever filmmakers try to pass as documentary in a narrative film--quite the contrary, in fact, as Winters amps up the volume on zooms in and puts in fast forwards, rewinds, and slow-downs, as though the raw footage is being watched and examined by a third party.

Slowly, we begin to understand what happened. Winters' screenplay has the same kind of zig-zag chronology as an Egoyan picture, utilizing a jigsaw construction assembled from the story's far-flung pieces. Between scenes of Sandra's confessions and denials, her son Stephen (Stephen Moran) lures a wandering toddler back to his grandfather's empty home. After a time, images of parents pleading for the return of their abducted infant appear on the television set. Stephen switches the channel. And then there are other scenes, flashes deeper into the past, horrible trespasses, secrets held.

Given those outlines, one could take a pretty good guess at how a story like this one would turn out, and in most cases, one would be right. But Snap is not interested in conventional outcomes and easy exposition. It is an unblinking look at everyday evil slowly uncoiling, in ways both expected and inexplicable, and it is permeated by a sense of awful things about to happen--or to be revealed.

It is given a firm, unyielding center by O'Sullivan's enthralling performance; she holds the screen with an intensity that tightens the entire picture. It is, in many ways, an actor's showcase role, but trickily so--there are countless opportunities to go over the top, to chew the scenery, and she craftily sidesteps each one of them. It's one of the most thrillingly nuanced performances I've seen recently; she's simply incapable of a false note, even when the script seems to be daring her to avoid one (name me one other actress who could pull off that gut-churning sex scene). The searing force of her performance is matched by the impenetrability of Moran's; through most of the film, he (and director Winters) stubbornly refuse to give us a clue what this kid's thinking, which plays like a thumb in the eye to our conventional need for rational explanations.

Winters, an Irish playwright making her film debut, shows a natural gift for cinema. Her framing is tightly composed but not over-controlled; her scenes don't feel staged, but nonetheless seem (in retrospect) designed for a specific effect. Even when her script occasionally falters, as in the reach for a too-clean dénouement in the closing scene, the series of shots and matching actions are quietly devastating. Most importantly, she doesn't let the dark, disturbing subject matter overtake the movie, to become its primary purpose. Snap is a dire, upsetting picture, but it is done with unquestionable skill and visceral power, and it got under my skin in a manner I wasn't fully prepared for.

Jason lives in New York. He holds an MA in Cultural Reporting and Criticism from NYU.