Much of the movie creates tension in not knowing what happened. Bernard-Pierre Donnadieu looks sufficiently creepy to be the villain, Raymond Lemorne, although the movie spends too much time showing us his private life where he seems perfectly normal. In the third act, Raymond confronts Rex and tells him that he will reveal what happened to his girlfriend provided that they go on a trip together.
This is where I have a problem with the story, because nobody in their right mind would get in a car with a sociopath. However, Rex is desperate to know what happened and reluctantly agrees to go. From this point on I think that the movie fails to follow any kind of real world logic, but the suspense and the performance of the actors makes it worth watching.
About two thirds of the film is told from the perspective of Rex and remainder is shown from the perspective of Raymond. This gives the film an interesting structure where their meeting seems inevitable.