Marie (Cecile de France) was the killer the entire time. The man we see committing the murders actually represents Marie's split personality. The entire movie was being told from her point of view, while she is in an insane asylum.
The film begins with a scene of Marie (Cécile De France) running through the woods, battered and bloody. It then cuts to her at a police station staring at a video camera preparing to give a statement.
We then go back in time as Marie awakens in a car with her female friend Alex (Maïwenn) at the wheel. The women go to school together and are driving to Alex's family farmhouse for a weekend of studying. There is no cell-phone reception there and thus, no distractions. Near the farmhouse, we see a Killer (Philippe Nahon) sitting in a steel truck, looking at the farmhouse. The Killer then drives into the nearby woods.
At the farmhouse, Marie is introduced to Alex's mother, father, and very young brother. Marie is given the upstairs room and the family begins to settle down for the night. Marie goes out for a smoke and sees Alex showering through an open window. Marie goes back to her room and begins to masturbate. As she does, the Killer drives his truck up to the front of the house and bangs on the door. The father answers and is quickly killed (as is the family dog). Marie hears the disturbance and begins frantically searching for a phone jack in her room to plug in a phone she finds in the closet, but is unsuccessful. The Killer confronts the mother and knocks her out before heading to Marie's room (where Marie successfully manages to hide under the bed after hiding all of her stuff). The Killer then goes into Alex's room (Alex is still sleeping as she wears ear plugs) and chains Alex up. Marie begins hunting through the house for a working phone, narrowly avoiding the killer on several occasions (while hiding in the closet, Marie witnesses the Killer decapitate the mother). Eventually, Marie discovers that the Killer has cut the phone lines.
The Killer throws Alex into the back of his truck. Alex's brother runs from the house crying for his mommy and is quickly apprehended and murdered by the Killer. In that time, Marie manages to sneak into the back of the truck too, giving Alex a butcher knife. Before she can release Alex, the Killer gets into the truck and drives off from the farm. They stop at a gas station and Marie manages to pry open the truck's back door and runs inside, asking the clerk for help. The Killer walks in and soon realizes that the clerk is on to him. Thus, the Killer murders the gas-station attendant with an axe, before again departing. Marie calls the police, who are largely unhelpful and unresponsive (partly because Marie doesn't know where she is or what the truck looks like from the outside). Feeling helpless, Marie takes the clerk's sports car and soon catches up to the truck. Marie follows the Killer's truck into a dirt path through the woods.
While in the woods, the Killer manages to get his truck behind Marie's car and crashes her car off a small berm. She crawls away into an abandoned greenhouse in the woods. The Killer sneaks up on Marie and almost kills her, but then begins to fondle Marie and ask why she is so desperate to save Alex. While the Killer taunts her, Marie grabs a hard object on the ground and bashes his head. While the Killer is dazed, Marie takes a stick wrapped in barbed wire and beats the Killer to death.
Meanwhile, the police have found the gas station with the dead clerk. The detective watches the video and IMPLAUSIBLE TWIST sees that Marie is actually the Killer (we see her murder the clerk with an axe on camera). Meanwhile, Marie goes back to the truck and unties and releases Alex, claiming to have "rescued her." Immediately upon being unbound, Alex accuses Marie of slaughtering her family and stabs her with the knife before running into the woods. Marie "reverts" back to the Liller persona and grabs a circular concrete saw from the truck before pursuing Alex.
Alex makes it to the road and flags down a car. The car stalls and Marie kills the driver and rips the car apart with the saw. Marie threatens a wounded Alex who attempts to crawl away. Marie approaches with the saw and drops it, kneeling down to tell Alex that she loves her and now nobody can get between them. The women begin to kiss, but while doing so, Alex stabs Marie with a piece of jagged glass she pulled out of her foot.
We then cut back to the police station, where Marie sits chained to the bed and ending her statement to the police. Alex watches Marie through a one-way mirror. A police officer assures Alex that Marie cannot see her. Suddenly, Marie smiles and throws her arms out as though to embrace Alex, who she can sense behind the glass.
The ending and twist are ambiguous. The first theory is that Alex seeing Marie shower caused her repressed lesbian feelings to snap her psyche. Marie invented the story to make her appear to be a hero (thus justifying her love for Alex), and the story was simply told from Marie's insane and unreliable point of view. The second theory is that Marie was crazy (some sort of multiple personality disorder) and truly did not know that she was the killer/that the killer was one of her personalities.
Order of Deaths:
Thanks Al (Pooper) and Evan B (Long)!