It is not immediately obvious why The Strangers is rated R. The horror film, about a young couple (Scott Speedman and Liv Tyler) terrorized in their home by a group of sadistic masked assailants, is reasonably violent, but not very graphic: aside from a brief glimpse of a pretty nasty gunshot wound and some stabbings that are either obscured or off-screen, there's not much here that would ordinarily raise rating board eyebrows. (The elaboration states that The Strangers is rated R for language in addition to violence, but there are, at the most, one or two muttered F-bombs.) In terms of content, PG-13 films have gotten away with worse. Hell, the PG-rated Chronicles of Narnia: Prince Caspian featured a decapitation, which is more than we see here. What, then, explains the MPAA's harsh (and no doubt economically damaging) treatment of The Strangers?

I am convinced that the film broke the R threshold in the eyes of the MPAA for one reason: it's pretty good. Tight, intense, often legitimately frightening, and committed to its suburban-nightmare premise, The Strangers may not be gory, but I wouldn't wish it on too many kids under fifteen. It's a classical, no-frills, 85-minute blast of cold air, a refreshing bit of professionalism in a genre whose mainstream, at least, has been plagued of late by lazy pandering and general shoddiness. Actually, The Strangers might be a good litmus test for your attitude toward the horror genre in its purest, most stripped-down form. It offers almost nothing in the way of plot -- the second sentence of this review is basically a complete rundown -- and even less if you're looking for an original story. (In fact, the premise and execution bear more than a passing resemblance to Them, last year's nifty French experiment in horror abstraction.) The characters are treated with some care, but there's nothing about them that's particularly interesting. As I mentioned, there isn't even very much here for the gorehounds. This is, strictly, the horror film as roller coaster, calculated to make you as nervous and uncomfortable as possible and to drop the floor out from under your feet every few minutes. Your enjoyment of The Strangers depends entirely on the extent to which you like squirming in your seat.

In terms of technique, Bryan Bertino's directorial debut is hardly groundbreaking, but it is notable for what it doesn't do. Specifically, Bertino keeps us off-balance by refusing to rely on cheap tricks. You know the cliché of the boogeyman appearing in the mirror when one of the heroes closes the medicine cabinet door? It's not here, and neither will you find much else like it. There's a scene where Speedman's James Hoyt is searching for something in his car, bending down several times to look under the seats and in the glove compartment, and each time I rolled my eyes, certain that one of the psychopathic villains would be standing there when the camera panned up. Instead, the film comes up with a much more amusing payoff. And while Bertino couldn't resist putting in an ostentatious false alarm, he only indulges in one, rather than the expected dozen.

The Strangers makes extensive use of sound design to create atmosphere and tension, but it doesn't punctuate its jump scares with cheesy orchestral stings that attempt to startle us out of our seats. The movie's peaks and valleys are much subtler – and scarier. Bertino knows that a quiet background movement, timed right, can work better than a character leaping into the frame. Some of the imagery he constructs here is genuinely chilling.

There's not much to the heroes or the villains, but there's enough to get the job done. As for the former, the film provides a brief set-up to give us some emotional context without trying our patience – we learn that James and his girlfriend Kristen have retreated to his family's summer home after a party where Kristen rejected James's wedding proposal. (The movie is actually quite efficient in the way it communicates this information through silences and painfully awkward moments.) Their three attackers wear incongruous masks (a sack with a smiley face; a doll; a pin-up) and seem to be in it solely for the thrill of tormenting their victims, their intentions toward James and Kristen pretty much aligning with the film's toward us. Their closest analogue is Michael Myers from the original Halloween – they're evil incarnate, with no apparent past and no motive beyond torture and killing. In keeping with its m.o., The Strangers doesn't linger on non-essentials.

The film is not without demerits. For one thing, it's laden with the useless and misleading "based on a true story" label, which raises questions and encourages scoffing rather than making the proceedings any more frightening. The final shot plays more like a homage to classic horror than something that needs to be in the movie; it's an unfortunate bit of excess in a film that's so effective largely because it's so spare. With the exception of a couple of individual images that might stay with you, The Strangers is probably too small and unassuming to make any sort of lasting impression. But all of that pales in the face of one fact: it works. It ties your stomach into knots. Indeed, it seemed to have been effective enough to earn an R rating.