An estranged twin wreaks havoc on a suburban family when she infiltrates their household under the guise of bonding with her sister, while her true goal is revenge and greed.  

Raised by a single alcoholic mom, Katie has learned to distance herself from others. A high school dropout who reaches her 18th birthday, Katie grows tired of the isolation she feels in her own dysfunctional home. It is time to rid herself of the one thing that bothers her the most - her mom.

After her mother's untimely death, Katie discovers adoption papers stating that she had a twin sister at birth who was adopted by their aunt. Emily was given the silver spoon with two loving parents raising her, along with her younger brother Matt, in an upper-class home. Envy sets in immediately. That life should have been Katie's, if only she had been chosen by the aunt instead of her sister.

Steve and Abby have a beautiful home in the suburbs. They have raised Emily as their own daughter, then had their son Matt years later. While short of perfect, the family is normal. Emily battles her parents on such things as her boyfriend, or curfew. Standard behavior for a girl who just turned 18.

One night, Katie arrives on their doorstep. The family's lives will never be the same. Emily grows upset that she was never told about being adopted. Abby feels for Katie but lacks an obligation to her, as they had been estranged Katie's whole life. Yet, due to her sister's demise, Abby believes she needs to help Katie, who has just been evicted from their apartment.

Katie works on a devious plan, to "bond" with Emily while studying her habits. Emily eventually gives in and even sort of likes having a sister after all these years. While Katie pretends to to fit in, she is actually sneaking her boyfriend Josh into the house, smoking weed, and plotting behind her aunt's back. After an elderly neighbor reports her to Abby, the woman suffers a fatal tumble down her stairs, courtesy of Katie.

Matt holds a secret he is afraid to tell - Katie extorts his lunch money from him daily. Once Abby discovers the truth, she demands that Katie leaves the house for good. Enraged over how she is treated by her new family, Katie enacts the plan that was brewing from the beginning... to kill all of them and assume Emily's identity so she inherits the small fortune. A makeover at a salon helps Katie to look just like Emily.

Katie forces Josh to hold Emily prisoner at his house while she deals with the rest of the family. As Katie sets her sights on each family member, Emily fights to free herself and faces a risky confrontation with Josh in order to rescue her parents and younger brother. Time is of the essence.