If anyone was paying even the remotest attention to little Oskar, they might notice that he’s developing some troubling proclivities. Though more perceptive, well read and unassuming than your average 12-year-old, the closest he ever gets to social interaction is the daily torments he silently endures at the hands of schoolyard hooligans. At home, he steals newspapers from his single mom to populate a scrapbook of obituaries and clippings concerning dreadful murders. He secretly carries a hunting knife around with him wherever he goes, occasionally caressing it when he thinks no one is looking. If these weren’t Columbiney enough behaviors to entreat a little dialogue (or psychotherapy), he frequently also can be found prowling the frozen courtyard of his dismal Swedish tenement alone after dark, stabbing at trees with all his scrawny, miserable might.

Yeah, if anyone at all was paying attention to little Oskar, they might notice that he’s a budding young serial killer.

Enter Eli. The unkempt little girl with depthless doe eyes and tousled hair who’s moved into the apartment next door startles Oskar on one of his jabbier evenings out, appearing noiselessly behind him on top of their courtyard’s jungle gym. Her conspicuous lack of disturbance over his screaming knifely activities is matched exactly by his thorough disregard for the fact that she got up there leaving nary a footprint in the snow. She tells them they can’t be friends, then promptly solves his Rubik’s cube for him. “I just twisted it till it was right,” she says. And a romance is born.

Turns out, though, the mysterious newcomer may have an ulterior motive. The aging gentleman who cares for her (and by “cares for her” read both: “loves her to death” and “slaughters innocent passersby like hogs to slake her undying thirst for human blood”) has been getting sloppy—and not only in the predictable way. One gets the impression from the meticulous preparations he makes for his nightly errands that he must have once been a genuine professional, but his mounting carelessness in his dotage has slid past clumsiness into flat out ineptitude. His propensity for public displays of evisceration inevitably leads to his capture, and Eli is on her own.

The exigencies of both Eli’s affliction and Oskar’s emotion are gradually, deliberately revealed, and as the awkward, tentative intimacy of their affiliation intensifies, so do the consequences of their actions. She apparently is much further down the path to complete human policy disconnect than her caretaker (Father? Lover? Acolyte? Slave? The history of their relationship is left tantalizingly ambiguous), and in her ravening, she doesn’t seem to give a bat’s ass about little things like, say, leaving witnesses or ropey gobs of evidence everywhere she goes.

Oskar’s demonstrated stabbitude and natural transparency combined with his desperate need for someone to be close to would seem a puzzle-perfect solution. Without one direct reference to this motive, she begins to coach Oskar in the finer arts of sociopathy from an intriguingly 12-year-old perspective. Though she casually informs him that she’s been 12 “for a very long time,” and unlike other tiny biters we’ve seen in film (think Ann Rice’s Claudia, or Homer from “Near Dark”) she seems wholly fixed in pre-adolescence, both physical and developmental.

Eli encourages Oskar—just as she is compelled to by her own insatiable hunger—to assert himself and his darker impulses on the greater world. The curse of his compulsions may seem relatively alien and upsetting to him, but she demonstrates that one’s nature, for better or worse, may not be an arguable point. Although this might at first seem like a point of liberation, the story goes on to offer some fabulously sophisticated interactions between Oskar’s “progress” and the revelation of Eli’s inability to control or enjoy her inescapable appetites.

The violence—and there’s plenty of it—though savage, visceral, unexpected and shocking in all the right ways, is more often implcit han explicit. Blood flows, limbs rend, bodies burn and rot and freeze, sure, but in the few shots that director Tomas Alfredson’s camera does bear immediate witness to the characters’ more feral moments, it’s usually from a great distance, or from behind frosted glass. Though each of their unspeakable acts lead to increasingly horrific repercussions, each is also soberly tempered with moments of delicate tenderness between the leads. The final confrontation, shot entirely in a submerged close-up of Oskar’s tragically serene face as he’s being drowned (not a spoiler, really), just might be one of the most sublime and startling illustrations of the frightful potential of true love ever committed to screen.

It’s hard telling if the film’s success owes more to the restraint of Alfredson’s touch or the elegance of John Ajvide Lindqvist’s screenplay, which he adapted from his own novel. The luminous severity at work in their choice of Sweden’s snowbound suburban backdrop, like a Vermeer painting or a haiku in its hushed simplicity, beautifully finds the ice in isolation, underscoring the longing, claustrophobia and alienation of virtually every character presented.

This is clearly a territory very short on warmth. The whole fable pointedly takes place in a gray zone—geographical, emotional, moral, even sexual. In a moment of painful fragility, Eli informs Oskar that she’s “not a girl.” He couldn’t care less. Their reciprocal acceptance of each other’s respective peculiarities is a uniquely compassionate balance of warmth and chills; a stoic, mature and appropriately elusive reflection on loneliness, confusion, monstrosity and adolescence in general.

As popular monsters go, the vampire may be fast becoming movieland’s new zombie, but this one is a cut above in every way, and will unquestionably slip in the back door as a classic of the form.

‘Let the Right One In’ screens at The Music Hall in Portsmouth Friday, Jan. 16, through Wednesday, Jan. 21, nightly at 7pm. For more information, visit www.themusichall.org.