Recently, I saw Haute tension a.k.a. High Tension.
I could follow the movie well until the weird twist in the end where we are shown that Marie and the killer is the same person (in Marie's body). I thought the ending was a bit incoherent as the movie couldn't support the idea of Marie having a second, murderous personality. Where did she find the track? How did she crash?
But a friend of mine told me that it is the other way around, Marie is in reality the serial killer, and all the other scenes with her as "Alex's friend" are in her imagination.
I like the second theory and it satisfies me more, but I don't think that this one can be supported by the scenes either. So, what really happened? Are there any further hints or evidence as to what was reality and what wasn't?