“Hard Candy” opens with a chilling scene of a modern sexual predator in action: an extreme closeup of the computer screen of the 14-year-old girl he’s trying to seduce online. The girl insists she’s more mature than he thinks. “I’ll have to see for myself,” he types back. Both parties remain unseen, and the anonymity of this exchange freights it with a powerful immediacy; it’s the most gripping moment of a thriller that plays resolutely, to both its credit and detriment, against audience expectation.

The man and girl meet up. As they flirt, Jeff (Patrick Wilson), a confident, good-looking fashion photographer in his early 30s, soon finds Hayley (Ellen Page), a short-haired gamine who audits her father’s med school classes and carries a biography of Jean Seberg in her backpack, to be a whole lot cannier than your average eighth-grader. Her nervous, uneven responses to Jeff’s practiced advances suggest she’s getting in over her head, but she hops into Jeff’s car to go to his place with barely a second thought.

As the pair pour screwdrivers and prepare for a photo session in Jeff’s living room, the direction things are heading seems so obvious that the abrupt u-turn the plot takes next doesn’t necessarily come as a surprise. As techno blasts from the stereo, Jeff begins to feel dizzy; when he comes to, he’s strapped to a chair. “Is this some teenage joke?” he asks, bewildered. “Teenage, yes,” Hayley says with icy certitude. “Joke, no.”

It’s a startling shift, and the opening gambit of a psychological contest in which Hayley and Jeff take turns as antagonist and protagonist, predator and prey. One of the film’s themes is how hard it is to know the truth about other people, and “Hard Candy” keeps the audience guessing about Jeff and Hayley until the very last moments.

Hayley holds an impromptu trial, with her own rules. She is certain Jeff has sexually abused underage girls; she’s also convinced he’s responsible for the disappearance of a friend of hers, and plans to extract his confession. She’s a tough cross-examiner: She accuses him, mocks him, shoves the evidence in his face, and sprays bleach down his throat if he tries to scream for help. She also goes through his e-mails and the hidden cache of porn to which she cleverly gains access. Jeff may crack under the weight of the evidence, the shame, and perhaps the humiliation of being brought to his knees by a girl he quite expected to have his way with. Then again, he may not, which is why Hayley has brought along some of her dad’s surgical tools.

The tension in the film is primarily psychological, but Jeff proves pretty good at untying himself (particularly when Hayley cuts him a break, as she does a few times, even leaving the room at one point to take a shower), and there are action sequences too, edited in the choppy, disorienting music-video style in which the director, David Harris, is clearly well-versed – perhaps too well-versed, in fact.These sequences are too flashy to be truly suspenseful. Similarly, the visual prominence of sleek minimalist-chic decor and bold monochromes – frames awash in moody tones of red or aquamarine, tableaux dominated by a single color, like pink or bright yellow – lacks a clear raison d’etre.The look of “Hard Candy” owes a debt to European art-house classics of the Pop Art era, notably Antonioni’s 1966 masterpiece “Blowup,” but that film’s exploration of the apathy and alienation linked to the modern fascination with image was well-served by its emphasis on aesthetics. That’s not the case here.

“Hard Candy,” a drama about two people butting heads, is not as viscerally compelling as it could have been. Despite the film’s commendable refusal to emblazon its combatants as good or bad, it still aims for the cathartic finish of a traditional revenge drama, and misses the mark. This genre operates on the assumption that the criminal deserves what he gets, and also that the vigilante is an essentially good person who’s only taken the law into her own hands out of necessity. (It’s never clear why Hayley, if she’s so certain of Jeff’s guilt, doesn’t simply report him to the police.)

Hayley, who begins as a promising character, degenerates into a one-note avenging angel as she gains the upper hand – she’s mentally imbalanced, sarcastic, and cursed with an annoying tendency to make bad jokes. Meanwhile Jeff, who spends much of the film in a state of Promethean impotence, gets to know humiliation, physical pain, and a swelling sense of regret – he develops, in other words, into an identifiable human being. To chart the courses of the characters this way, in the name of keeping things unconventional, seems a fundamental miscalculation.