A deranged woman who gave her baby up for adoption years earlier becomes the teen’s best friend in order to turn her against her parents and destroys anyone who gets in the way of their relationship.

When Alex goes to leave his psychotic girlfriend Mia, she tells him she is pregnant but he believes it to be a lie to keep him in a relationship. Mia assaults Alex rather than let him go and is sent to prison while Alex moves on with his life.

Soon after the incident, Alex meets Megan and falls in love. They marry a mere few months after the break-up. Megan works at an adoption agency and falls in love with a baby girl needing a forever home. Alex desires a family as well so the couple adopts baby Zoe.

Nearly 16 years later, the family is doing well. Zoe is a typical teen who looks to her parents for everything while striving for independence. In dance class one day, a new student arrives. It’s Mia with a different hair color. She has located the daughter she had given up while in prison and wants to be a part of her life. The fact that Zoe’s actual birth father has unknowingly adopted his daughter is a bonus for Mia.

Zoe sees something in her new friend Lacie (AKA Mia) that she’s missing from her own mom. Mia listens, and gives her much more support and independence. Mia takes Zoe on a shopping spree, where she dotes on her daughter by buying her new clothes, getting her hair done, even letting her have her ears pierced, something Megan refused to do. To top it off, Mia buys Zoe a new phone because her mom’s hand-me-down was barely working and Zoe’s parents tell her to get a job to buy a new one.

When arriving home after the shopping trip, Zoe’s parents catch her sneaking in. They don’t know who bought her these things but demand she return the clothes. Zoe hides the new phone from them. Their interference only alienates Zoe from her parents more. She complains to Mia, who seizes the opportunity to drive a bigger wedge between her daughter and the couple who raised her.

Zoe’s best friend Casey is suspicious of Mia, causing Mia to worry enough that she kills Casey. When Zoe’s new boyfriend Trevor starts to take her attention away from Mia, he finds his water bottle poisoned at football practice. Even Zoe is starting to worry a little because of Mia’s irrational actions at times. When Megan buys Zoe a new phone so she can return hers to Mia, Zoe brings everything Mia bought her to dance class to give back to Mia, only she isn’t there. Zoe finds her address and visits her house to return the things, but she is subdued through chloroform and taken to a cabin that Mia’s family owns.

Alex arrives home from a business trip the same day Zoe goes missing. The police won’t help until 24 hours are up. Megan cleverly got a picture of Lacie from Zoe’s phone and shows it to Alex. He tells her that’s his ex Mia. They realize now what has happened and tract Mia to her cabin. Alex goes in first but is subdued by his ex. Megan is forced to sneak in and battles against Mia. Alex is stabbed in the process, and Megan succeeds in taking Mia down to save her family.