To indicate exactly how bad High Tension is will require what we now call "spoilers," although as one critic noted, the movie itself is already beyond spoiled in the traditional sense of the word. It is, in fact, rotten.
A French slasher flick walking a fine line between incompetent homage and lazy theft of Texas Chainsaw Massacre, High Tension centers on a law student named Marie, who takes a trip with her friend Alexia out to the friend's family farm in rural france, where they'll study for what I assume is the French equivalent of the bar exam. It becomes obvious fairly early on that Marie has an unrequited crush on Alexia, and that Alexia is completely oblivious.
But first: what's this, as the girls arrive at the farmhouse? Why, it's a filthy, heavyset figure in an old rusty truck, having some sort of sexual congress with a severed female head, which he dumps out the window when the task at hand reaches completion!
Subtle. I'm guessing a lot of people are going to stop the disc right there, but I've watched Audition. I've seen worse.
Before long, of course, the killer breaks into the house for no apparent reason, butchers Alexia's family, and kidnaps her. Marie narrowly escapes detection and sets off to rescue her friend. At this point, High Tension is derivative and a little strained, but not beyond the horror films it so obviously apes. Director Alexandre Aja knows his way around a camera, even if he doesn't have a lot of original ideas--there's the obligatory gas station scene, the unhelpful phone call to the police, and a power tool straight out of Chainsaw.
Where the movie goes horrifically wrong, and where I will be spoiling what little narrative creativity that High Tension boasts, is in its "twist ending." See, once Marie manages to catch up with the killer, gruesomely dispatch him, and rescue her would-be romantic interest, it is revealed that she was the killer all along. What had appeared to be a flawed but sympathetic description of a strong lesbian protagonist turns out to be a sociopathic sexual deviant.
Even ignoring the gay-bashing incongruity of this Fight Club ripoff, it's just incredibly poor writing. Although the "Keyser Soze" reveal has been around for decades, it still manages to work in films that use it as the final piece of a puzzle, causing viewers to say "ah-hah! now it all makes sense!" Whereas High Tension's twist actually destroys what little narrative coherency that it had left. If she's the killer, then where did that big, rusty truck come from? How does she drive two cars at once? Why did the gas station attendant (and indeed, every other character) treat the killer as an entirely different person? Who called the cops? And why did the psycho spend so much time hunting through the house for her, when there wasn't anyone else to hunt for?
We could try to come up with psychological explanations for these glaring plot holes, but it hardly seems worth it. Clearly, the filmmakers didn't make the effort. I'd recommend you do the same, and leave this one unwatched.