SYNOPSIS
The film opens with Julia Banks (Rosario Dawson) in an interrogation room, being questioned in the murder of her psychotic ex-boyfriend, Michael. Julia claims that she has been set up and that she has nothing to do with the murder, despite the police's copies of instant-message conversations between them clearly showing Julia lusting after him.
The film cuts to six months earlier, with Julia leaving her job as an editor at a popular fiction website to work from the home she will be sharing with her fiancée, David Conover, trying to settle into her new role as a wife and a stepmother, she believes she has finally met the man of her dreams, the man who can help her put her own troubled past behind her. Julia's friend Ali buys her some presents and tells her that if she needs anything, she is there. Julia then drives to David's mansion, where she is greeted by David's ex-wife, Tessa (Katherine Heigl), who is barely coping with the end of her marriage. After saying goodbye to her daughter, Lilly, Tessa leaves, but not before shooting Julia a dirty look. That night, David and Julia have sex.
The next evening, Tessa shows up at the house to drop off something, but Lilly insists she stays for dinner. While they are eating, Tessa openly criticizes Julia's cooking and leaves. Julia then gets a notification that her restraining order against her abusive ex-boyfriend, Michael, has expired, much to her distress. That night, she sees a dark shadow walking outside the living room window, but it just turns out to be David, home from work late.
A few days later, Tessa's controlling mother comes to visit, angry at her for causing her marriage with David to fall apart. She berates her and then goes outside to watch Lilly play, while Tessa sits inside, crying while watching her and David's wedding video. She then puts Julia's name into an identity search engine and discovers her restraining order against Michael and sets up a Facebook account for Julia and begins messaging Michael, pretending to be Julia, telling him how much she misses him. That night, at a grand opening party for David's brewery, Tessa snoops on Julia's phone while she's dancing with David and discovers that the two are planning to be married. Enraged, she storms out of the place and has sex with one of the bartenders.
The next day, Julia is getting ready for a bath when she hears the doorbell ring. While she's downstairs checking, Tessa breaks into the house and steals David's watch, wedding ring, and a pair of Julia's panties. The doorbell was rung by a deliveryman, carrying flowers sent anonymously by Tessa to make it look like Julia is having an affair. Julia angrily slams the door and goes back upstairs. That afternoon, Julia takes Lilly to a farmer's market but she runs away while Julia takes a phone call. After a few minutes of frantic searching, she finds Lilly in Tessa's arms, who is complaining to David about the incompetence of Julia's role as a mother. Tessa shows up at the house again sometime later and Julia yells at her about trying to get back into her husband's life. After a brief argument, Tessa drives away. That afternoon, the two have lunch together to try and reconcile, and Tessa reveals the reason that she left David was because he was having an affair. That night, Tessa mails Michael the watch, ring, and panties.
A few days later, Tessa attempts to get Lilly to ride an unruly horse at her riding practice. Julia, who is also there, notices how scared Lilly is and convinces her to come down and takes her home, much to Tessa's annoyance. That night, Julia sees David flirting with another woman at a dinner party, and the two have heated sex in the bathroom, interspersed with scenes of Tessa pleasuring herself while she messages Michael about how much she wants him, pretending to be Julia.
Tessa cuts off Lilly's long hair as a punishment for leaving riding practice early, which Julia notices when Tessa drops her off that afternoon. The two have a heated argument on the stairs before Tessa throws herself down them and pretends that Julia pushed her when David arrives a few minutes later. He takes her to a hospital, comes back home and castigates Julia for treating his ex-wife so badly. The next day, Julia calls Ali over to investigate Tessa's past, and the two discover that she has history of violent and unpredictable episodes.
After a series of increasingly heated meetings between the two women, Tessa finally messages Michael, asking him to come to her. Michael breaks into the house while Julia is making breakfast, and she is noticeably upset at his presence. He then attacks and tries to rape her, but she stabs him in the leg with a kitchen knife and escapes the house. Tessa, who was waiting outside the whole time, puts gloves on and enters the house, killing Michael by stabbing him in the heart.
The police are notified, and Julia is taken into questioning, but they release her due to circumstantial evidence. David finds out about Michael and a tearful Julia explains her relationship with him. David heads to Tessa's house to see Lilly, but after seeing burned gloves in the fireplace, realizes that Tessa set up Julia for the murder. She knocks him out with a fireplace poker and goes into the kitchen to get a first-aid kit. Julia realizes what has happened and drives over, getting Lilly out of the house while Tessa is busy in the kitchen. Julia tries to call 911, but is discovered by Tessa and the two fight in the hallway, ending with Julia smashing Tessa's head into a glass picture frame, apparently knocking her out. She crawls into the living room to revive David, but is surprised by Tessa, who wasn't knocked out, holding a knife. She tries to kill Julia, but after looking in the mirror and realizing the monster she has become, stabs herself in the heart. With her last words, she begs Julia to not let her daughter remember her like this. David wakes up as the police arrive.
Six months later, David, Julia, and Lilly have adopted a dog and moved into a new house, apparently moved on from Tessa's rampage. There is a knock at the door and Julia opens it, revealing Tessa's mother, wanting to see her granddaughter. Julia looks on in horror as she hugs Lilly, promising to make things the way they used to be.
