Poster for Let The Right One InHorror is difficult to do well, because it is a genre poised precariously on the precipice of comedy. The worst horror films will elicit screams of laughter, rather than terror, because the mind would rather cover sights of murder, bloodshed and gore with incredulous laughter than accept them at face value. Newsreels from war zones are too real for this reaction, perhaps because they show too little of their sickening subject matter. Thus, a bad horror film provides its audience with an outlet for any kind of pent up morbidity. Tomas Alfredson, director of Let The Right One In, takes a gamble, then, with the scene toward the beginning of the film in which he depicts a snow-white poodle innocently drinking the blood of a murder victim. It's a hair-trigger moment in a movie full of them, and it's played with such a surreal straight face that the audience can't help but be drawn inexorably in.

The murderer is a man named Håkan, an almost (but only almost) laughably incompetent butcher whose task is to find human blood for his weak young vampire charge, Eli, who is played with astonishing poise by 13-year-old Lina Leandersson. The actress's voice was dubbed with a throatier tone, but her performance is all in her face, which shows a mixture of consternation and lost innocence that is surely far beyond her years. As with most of the film, it is the unspoken and unseen that is most powerful. Alfredson masterfully contrasts the mundane with the horrific, but within that conflict there exist beautiful vignettes of suburban boredom - the flailing marriage, the man who lives with nine cats and the father more interested in drinking than his son. None of these people are ever judged or condemned: the camera merely observes them with as much dispassion as it does the death of Håkan, the murderer, and the burgeoning relationship between 12-year-old Oskar and the undead Eli, which forms the spine of the film.

The special effects let the piece down only once, with the bad animation of said cats, in a certain sequence. However, it is a testament to their effectiveness that throughout the remainder of the film they are entirely invisible. The rest - the scaling of buildings, spontaneous combustions, and decapitations - all occur with a simple grace; they are used not to augment the story, but to progress it to its perfectly shaped conclusion.

The makers of Let The Right One In have realised that, with comedy on one side, horror is also surrounded by sadness. The best horror films - Don't Look Now being the perfect example - are as achingly sad as they are frightening. Once again Let The Right One In matches and exceeds this quota in its portrayal of Oskar and Eli's relationship that can never, for so many reasons, be consummated. This is melancholy in and of itself, but also sad due to its oblique hints toward its inevitable conclusion. Despite this, there is enough light to make the darkness genuine, and there is never even a hint of popularist emotion for its own sake. The film performs a cunning balancing act, between the contrasts of horror, comedy and sadness, all of which are present to just the right degree. This is reflected in its desolate visual design - the white of the Swedish snow forever slashed through with the branches of dead trees and the shafts of light from apartment windows (Alfredson even avoids the undeniable temptation of the "red-blood-on-snow" shot). Surely Let The Right One In is the best horror film that will be released this year, if not the best film altogether.