Maria Brennan (India Eisley) is a timid social outcast at her high school, where she is constantly bullied by her peers led by her schoolmate Mark (John C. MacDonald). Maria has only one friend, Lily (Penelope Mitchell) whom she envies, and for whose boyfriend, Sean (Harrison Gilbertson) she harbors a secret crush. At home, Maria frequently suppresses her emotions with her parents: her father Dan (Jason Isaacs), a philandering plastic surgeon and a perfectionist; and her mother Amy (Mira Sorvino), who suffers from depression and pretends to be oblivious to her husband's affairs.
Maria accidentally discovers a sonogram of a pair of twins and starts hearing voices from her mirror reflection, Airam, who is more beautiful, charismatic and aggressive. Maria is initially frightened but eventually finds solace in Airam's empowering talks that make her confront her subconscious thoughts and inner feelings. After being deserted by Lily – who was aware of Maria's feelings for Sean – and humiliated by Mark at the prom, Maria agrees to swap places with Airam in exchange for Airam's help in solving her problems.
Airam begins to seek revenge on people that have wronged Maria and take control of every aspect of her life that she was unhappy with. She lures Mark to the shower and breaks his knee, and arranges for Amy to run into her husband's mistress as a way of forcing her to acknowledge Dan's affairs and their superficial marriage. Airam also secretly practices figure skating and pursues a disconcerted Lily across the ice, resulting in Lily's death from crushing her skull on the pavement. Ignoring Maria's (who is now trapped as Airam's mirror image) protests and pleas, Airam proceeds to seduce Sean, who eventually gets suspicious and tries to leave, only to be bludgeoned to death by Airam.
Airam confronts Dan at the clinic after hours, pretends to be heavily intoxicated, strips down naked and demands to know if he would still love her if she were not perfect. Viewers are reminded of an earlier scene where Dan offers to operate on Maria in order to make her “perfect”. It is revealed that Maria originally had a twin sister (presumed to be Airam at this point), who was put down after birth by Dan due to her physical deformities. Having obtained his answer, Airam slits Dan's throat with a scalpel.
Airam now no longer sees Maria in her reflection as she was previously able to, leaving the audience to speculate that Airam is in fact a manifestation of Maria's subconscious and that the two of them have come together as one, with Maria now possessing Airam's traits. A series of mirrored burst shots depicts Maria and Airam together with their mother reunited as a family, presumably now merged into one.