A woman sets out to make life a living hell for her ex-husband's new fiancée.
Julia Banks is being questioned by a detective about 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 social-media conversations clearly showing Julia lusting after him. The scene then cuts to six months earlier, with Julia getting a send-off from her boss and friend Ali, from her job as chief online editor for a storytelling website. Julia is now going to work remotely from the mansion she will be sharing with her new loving fiance, David Connover, and her soon-to-be stepdaughter Lily. Arriving at David's, she is greeted by David's ex-wife, Tessa, who is still not coping with the end of her marriage two years earlier..
Julia Banks is questioned for the murder of her psychotic ex-boyfriend, Michael. Julia claims she has been set up, despite social media conversations showing her lusting after Michael.
Six months earlier, Julia receives a send-off from her boss and friend, Ali, from her job as chief online editor for a storytelling website. Julia plans to work remotely from the mansion she will share with her fiancé David Connover and her soon-to-be stepdaughter Lily. At David's house, Julia is greeted by his ex-wife Tessa, who is not coping with the end of their marriage two years earlier.
The next evening, Tessa drops something off at the house, and Lily insists she stays for dinner. Tessa criticizes Julia's cooking and leaves. Julia learns that her restraining order against Michael has expired, much to her distress.
Tessa is visited by "Lovey", her controlling and belittling mother. Investigating Julia online, Tessa discovers her restraining order against Michael. She sets up a Facebook account impersonating Julia, and messages Michael, telling him how much "Julia" misses him.
Julia receives a surprise flower delivery, sent anonymously by Tessa to make it appear Julia is having an affair. While Julia is at the door, Tessa sneaks in and steals David's watch, his grandmother's wedding ring, and a pair of Julia's panties. That afternoon, while Julia and Lily visit David at a farmer's market, Lily wanders off. Julia finds her in the arms of Tessa, who is complaining to David about Julia's incompetence as a mother. When Tessa shows up at the house, Julia confronts her for trying to reenter David's life. The women have lunch together in an attempt to reconcile, and Tessa claims that David left her was because he suspected she was having an affair. Tessa mails Michael the watch and panties.
Tessa tries to get Lily to ride an unruly horse at her horseback riding practice, but Julia notices Lily's fear and takes her home, to Tessa's annoyance. That night, Tessa pleasures herself in her car while sending sexually explicit messages, posing as Julia, to Michael.
Tessa cuts Lily's long hair short as punishment for leaving riding practice early, leading to a heated argument with Julia. Tessa throws herself down the stairs and pretends Julia accidentally pushed her when David arrives. After taking Tessa to a hospital, David castigates Julia for treating his ex-wife so badly. Julia calls on Ali for help, and they discover Tessa's child services records; as a teenager, she attempted to kill her father when he cheated on her mother.
Tessa messages Michael, as Julia, saying she wants him to visit for sex. Michael enters the house when Julia is alone, and when she spurns his advances, he attacks her. A badly beaten Julia stabs him in the leg with a kitchen knife and escapes the house. Tessa, waiting outside, enters and stabs Michael in the chest.
In the present, the police release Julia, stating the investigation into Michael's death will continue. David is initially supportive after a tearful Julia explains her history with Michael, but when the police show him the "evidence", he becomes disillusioned and goes to Tessa's house to get Lily. There, he sees burned gloves in the fireplace and his grandmother's wedding ring on Tessa's finger, realizing that Tessa framed Julia.
Tessa knocks David out with a fireplace poker. Julia arrives and tries to call 9-1-1, but is attacked by Tessa. Subduing Tessa, Julia rushes to revive David, but Tessa attacks her with a knife. Seeing herself in a mirror, Tessa realizes the monster she has become and stabs herself in the stomach. Dying, Tessa begs Julia to not let Lily remember her like this.
Six months later, David and Julia are married and have moved into a new house with Lily. Julia answers the door to find Lovey wanting to see her granddaughter. Julia looks on in horror as Lovey hugs Lily.
