Whenever something terrible happens, some persons just have the specific coping mechanism that block them from remembering anything that happened. That is what happened to young teenage girl Anna Rydell (Emily Browning) on the night when her mother died in a fire accident. After her death, Anna tried to commit suicide and ended up in the mental institution. Ten months have passed and she still can't remember anything, and that bothers her. But, since her psychiatric Dr. Silberling (Dean Paul Gibson) is more than certain that she is fine and has resolved all her issues, with this memory lose not being as bad as she thinks, he discharges her from the hospital. Her father, Steven (David Strathairn) comes and picks her up. It is only when she gets home that she finds out that her mothers former nurse, Rachel Summers (Elizabeth Banks) has become her new stepmom and had moved in a while ago. She also gets to see her sister, Alex (Arielle Kebbel), who reveals how she sent letters and CDs, but Anna never got any. At one point, Anna starts having strange dreams and hallucinations that are so real, she even has some bruises to prove how real they are. And two of them start suspecting that Rachel isn't who she claims to be, but a murderer who is taking someone else's identity and they are the next victims. Will they manage to bring her to justice or is it another mental illness?