The ending of this movie makes no sense. Everything up until the end is good, but end itself is so profoundly stupid it manages to ruin the entire film.
Alex and Marie are friends from school. They drive to Marie's house in the country in order to spend some time away from the distractions of the big city and get some studying done. The same night they arrive at Marie's home, a stranger attacks the farmhouse, killing Marie's family but sparing Marie. Alex manages to allude the killer and hides in his truck. When the killer pulls over at a gas station, Alex tries to get help from the attendant but the killer thwarts her plan. Stealing the attendant's car, Alex follows the killer and the two finally face off in the woods.
Alex is the killer. She's got MPD and, though it defies all logic, both personalities seem to exist at the same time. The way the film is made, people interact with both Alex and the killer, who manifests as a middle-aged man, while the two operate in the same space and time. Given that the film is set in this world and thus follows the rules that govern our reality, Alex being both the killer and the heroine is impossible. That's not to say that other movies about MPD killers are logically flawed, it's just that the way in which this story is told, what takes place on screen can't acutally be happening. But since the film insists on creating a concrete visual reality, it completely fails in its attempt to tell a compelling story.
The rest of the movie is, as I said, pretty good. The gore isn't as extreme as in later French New Wave genre films, but the brutality of the murders and the cold single-mindedness of the killer are horrific in their own right. Were it not for the unfortunate ending, and the rather puzzling but no less disgusting scene at the beginning of the killer fallating himself with a severed head, High Tension could have raised the bar for future French gore films. Instead, it ushered in a new generation of films in which women are repeadedly both villains and victims of horror.