Sara Matthews (Minka Kelly) is starting her freshman year of college at ULA. Upon arrival, she meets Tracy Morgan (Alyson Michalka), an avid party-goer, and Stephen Morterelli (Cam Gigandet), a drummer who plays in a band for a fraternity who later begins to date Sara. After a party at Stephen's fraternity house that lands Tracy drunk, Sara arrives back at her dorm late at night and meets Rebecca Evans (Leighton Meester), her new roommate.



Initially, the girls begin to bond very well as Rebecca learns more about Sara's life. Sara had an older sister, Emily, who died when Sara was nine. She keeps a necklace that belonged to Emily as well as her sister's name tattooed on her chest. She also has an ex-boyfriend, Jason (Matt Lanter), who keeps calling her to try to get back together with her. The history behind their breakup is that Jason and Sara agreed to both go to Brown University or not, and when only Sara got a spot, she declined it and chose ULA. However, when Jason got accepted to Brown at the last minute, he went without Sara, reneging on their agreement. Against dorm regulations, the two girls secretly adopt a stray kitten which Sara found.



As time goes on, though, Rebecca begins to become more obsessed with spending time with Sara. She is willing to keep at bay anyone who wronged Sara or who she thinks is stealing Sara away from her. This is evident in multiple scenarios:



– Tracy's avid party-going takes a turn for the worse when she invites Sara to a dance/bar club but leaves her there, who eventually calls Rebecca for a ride. Later, one night, Rebecca jumps Tracy in the shower and threatens to kill her unless Tracy stays away from Sara (she even pries Tracy's belly ring from her bellybutton). This leads Tracy, now fearful, to move to another dorm.



– When Sara leaves her phone in her room one time to go out on a date with Stephen, Rebecca hears it ring and answers only to find it is Jason on the other end. Impersonating Sara, she tells Jason never to call her (Sara) again. For several weeks afterwards, Jason does not call Sara, much to the latter's amazement.



– When Sara's philandering interior design professor (Billy Zane) makes a pass at her in private, Rebecca goes into his office and tempts him to make a pass at her, while recording their dialogue on a tape recorder to make it look like he was hitting on her. Sara, oblivious to what Rebecca has done, is surprised to find out the next day in class that she has a new professor, Chanel DuBouis (Sharon Leal).



– When Irene (Danneel Harris), one of Sara's friends who already has her own fashion design, invites her to move in to her place so she can keep their stray cat, Rebecca kills the kitten by putting it in a laundry room dryer and pretends that it had run away.



- When Stephen invites Sara to spend Thanksgiving with him, Rebecca beats herself up to look like she went looking for their cat but was assaulted by a street thug. Sara, feeling bad for Rebecca, decides to spend time with her at her parents' place.



Rebecca's folks (Frances Fisher and Tomas Arana) are delighted and surprised to see their daughter, who didn't announce that Sara was coming. During her stay, Sara overhears a conversation between Rebecca and her father; apparently Rebecca has had a troubled past with making friends. Later, her mother, in a conversation with Sara, hints at medication that Rebecca is supposed to be taking. When the girls go out to a coffee shop, they bump into a girl, Maria (Nina Dobrev), who was acquainted with Rebecca from high school and is on some of Rebecca's portrait drawings. Maria tells Rebecca with unease, "We were never friends."



After Thanksgiving break, Sara learns that Irene has returned from a fashion tour. When Rebecca goes to Irene's studio, Sara and Stephen find from her drawers a bottle of pills of Zyprexa, which is used to treat schizophrenia and bipolar disorder, but, as the bottle is full, she has never been taking them.



At the studio, Rebecca and Irene meet, the latter unaware that Rebecca is roommates with Sara. Irene, who is somewhat attracted to girls, shares a romantic moment with Rebecca, and the two go back to Irene's place. Finally, Rebecca draws the last straw when she gets a tattoo which she reveals is identical to Sara's with her sister's name. Emotionally shocked, Sara runs off. She packs some of her things, with the exception of her sister Emily's necklace that she can't find, and moves in with Stephen at his fraternity house. Later, Stephen and Sara find one of Rebecca's books in which she has handdrawn portrait sketches of Sara, just like the ones of Maria. The implication is that Maria was once a 'best friend' that Rebecca has obsessed over, but now Sara has taken her place.



In the midst of all this, Jason has arrived on campus at Sara's dorm. He leaves a written note under her door revealing his desire to see her as well as the hotel and room where he is staying. Rebecca, having read the note, poses as Sara with her sister's necklace, the tattoo she got, and the newly dyed hair, goes to Jason's hotel room, and stabs him to death with a pocket knife.



Eventually, all scenes converge when Sara gets a text from Irene's phone saying she needs her right away. She in turn leaves voicemail on Stephen's phone telling him she will be at Irene's place. When she gets there, she finds Irene held hostage by Rebecca with a pistol. She reveals to Sara that she was responsible for what happened to Tracy, the professor, Jason, and the kitten and that she did it all because she wanted to win Sara's friendship. When Sara apologizes, Rebecca forgives her but wants to kill Irene. The two begin fighting, and Stephen arrives just in time to help stop Rebecca from pulling the trigger on Irene. However, in the ongoing struggle involving gunshots fired, Stephen is knocked unconscious while Sara is left dangling out of the window high above the ground (she had been trying to escape to get help). Although Rebecca pulls Sara back to safety, the latter reaches for the pistol with which to shoot her, but the cartridge is empty. Rebecca, enraged that her so-called friend would shoot her, attacks Sara with insanity and attempts to strangle her. Sara reaches for a pocket knife and, saying, "You were never my friend," (echoing Maria's words from earlier in the film), stabs Rebecca in the back, effectively killing her. She retrieves Emily's necklace back from her.



Life on campus goes on, as Sara moves back into a dorm. With Stephen's help, she moves the extra bed set out of her room, proclaiming that she does not want a roommate for a while.