A disturbed young woman gets obsessive about her female college roommate. Sara Mathews (played by Minka Kelly) is an ordinary student going to a university in Los Angeles to study fashion design. Her dorm roommate is an art student named Rebecca (played by Leighton Meester). At first they are just friends but as time goes by, Rebecca gets more and more possessive of Sara. Sara makes another female friend Tracy (played by Ali Michalka). They go to a party together. This makes Rebecca very jealous. Sara's professor in fashion design class is Professor Roberts (played by Billy Zane). The professor makes an inappropriate pass at Sara which she rebuffs. Later she tells Rebecca about it.
Sara finds a stray kitten. She brings it back to the dorm room to show Rebecca. They decide to keep it, even though pets are not allowed in the dorm. Rebecca shows Sara some of her favorite art by another artist. The images make Sara uncomfortable but she doesn't say anything. Tracy drops by and invites Sara and Rebecca to go to a night club. Sara agrees to go but Rebecca stays in the dorm. While Sara is out Rebecca's goes through Sara's things, tries on Sara's necklace that she was explicitly told by Sara never to wear, and puts on some of Sara's perfume. Rebecca makes Sara cancel a date with a boy, named Stephen (played by Cam Gigandet) to be with her instead.
Later, while Tracy is alone in the dorm showers she is viciously attacked. We cannot see who the attacker is, but we realize is was Rebecca. Tracy moves to another dorm building because of this. When Sara comes home very late with Stephen, Rebecca screams at them because Rebecca was so worried about Sara. Sara tells Rebecca that she has no reason to worry about her. The next time Sara goes with Stephen, Sara goes with him back to his place. Rebecca goes out of her mind with worry when Sara does not come home. Sara has left her phone behind and Sara's ex-boyfriend Jason calls it. Rebecca knows who he is and Rebecca answers the phone pretending to be Sara. She engages in phone sex with him, then tells him never to call again. When they get caught by the dorm monitor having an unauthorized pet, Sara tells Rebecca she may move out to live with her other friend Irene off-campus, so that Sara can keep the cat. Rebecca then kills the cat by putting it in a dryer in the laundry room and turning the machine on.
Rebecca injures herself on purpose, then tells Sara she was attacked in order to get Sara to pay her more attention. Rebecca makes Sara promise to always be with her. Rebecca also gets rid of the Professor that is hot for Sara by blackmailing him. One-by-one Rebecca eliminates anyone or anything that comes between her and Sara. When Sara goes with Rebecca to meet Rebecca's parents Sara learns from Rebecca's mother that Rebecca has severe mental problems and she should be taking medications for it, which she is not doing. The only person left that Rebecca needs to remove is Sara's new boyfriend Stephen. Rebecca dies her hair to look like Sara's and late at night sneaks into Stephen's bed pretending to be Sara. When he awakens she cuts him with Sara's utility knife that Sara uses for making her fashion designs. Stephen is not convinced that it was Sara that attacked him so the plan backfires.
Later, when Stephen and Sara do not meet as planned, they both suspect the worst. Stephen knows Sara was going to see Irene. When Sara enters Irene's house she sees that Rebecca has tied her up. Sara tries to calm Rebecca, but Rebecca is out of her mind. Rebecca tells Sara she loves her and that is why she got rid of the ex-boyfriend, the professor, her friend Tracy, killed the cat, stabbed Stephen and now is getting rid of her friend Irene. Stephen arrives just in time for the final violent showdown.
Best part of story, including ending:
It is intriguing that at first Rebecca just seems a bit odd, and to see how she becomes more and more psychotic as the movie progresses.
Best scene in story:
The shower scene is very dramatic and scary when Rebecca attacks Tracy. It is reminiscent of the famous shower scene in Hitchcock's movie Psycho.
Opinion about the main character:
Sara actually likes Rebecca very much and if Rebecca was not a crazy nutcase, they might have become very good friends. I like that Sara is not at all judgmental of Rebecca's behavior until it gets way out of control.