Katie (in a cheap brunette wig) (Nicky Whelan) is holding a crying baby and fighting with a man, presumably her husband, and stabs him with a kitchen knife, killing him. Four years later we come upon Angela (Gina Gershon) and Brian (Nicolas Cage) who have a four-year-old daughter named Cora. The three are a happy, close family, with Brian working as a Doctor and Angela on leave from her job as a Doctor to be a full time mom to Cora. A friend of Angela's, Linda (Natalie Eva Marie), who runs a Mommy and Me gym, introduces Angela to Katie (now with her real, blond hair).Katie's four-year-old daughter Maddie hits it off with Cora. Katie opens up that she left an abusive husband but doesn't elaborate. She also refuses to have her photo taken. Brian's mother, Donna, (Faye Dunaway) doesn't like Katie from the start and questions her background, sensing something is off about her. Angela confides to Katie that she had trouble conceiving so used an egg donor for Cora and is now pregnant again.She miscarries and is devastated and invites Katie to move in with Maddie into the guest house and be a part time nanny for Cora while she recovers. One night Brian is looking out the window when Katie steps naked into the pool (knowing he is watching). He looks uncomfortable and moves away from the window. Angela decides to go back to work at the hospital part time to give her more of a purpose and Katie watches Cora. Katie uses her free time in the house to go through Angela and Brian's belongings, trying on clothes and snooping, showing obvious envy of their family.Angela comes home after work one day and catches Katie having sex in the guest house with whom she thinks is Brian but is actually a woman (later revealed to be Linda). After the initial embarrassment this makes Angela relax a bit and trust Katie more, not believing she would ever have any interest in Brian.Linda tells Katie that Angela is going to ask (Linda) to be their surrogate and is excited about it but Katie can barely hide how it upsets her. Apparently, Angela has one embryo left from the same donor that Cora was conceived from so wants to use that so the two can be full siblings. Katie confides to Linda that she is the donor, and that her daughter Maddie was also hers but born from a mother in Maine. Katie abducted Maddie as a baby because "the mother didn't take care of her" and now she worries that Cora isn't getting enough attention either and wants to take her.She drowns Linda in a lake and her body is later found by a couple. Four months later Katie is carrying Angela and Brian's baby (and unknown to them, her own baby). Donna questions Angela in choosing Katie and why she picked her and what does Katie get out of it. Angela says Katie will be paid forty thousand dollars and that she doesn't expect anything else.It is revealed in a flashback that Katie, whose real name is Tara, got an infection in both ovaries and can no longer have her own children. As Katie's behavior gets stranger and more delusional - Angela catches Katie lying to a mother in a park about both girls being her twin daughters - Angela becomes more suspicious.Brian comforts Angela and says everything will be fine when the baby comes, and Katie will move on with her life. A couple months later Angela and Brian tell Katie they are getting a full time Nanny and Katie accuses them of watching a complete stranger to raise their child and angrily leaves the table. Brian defends her and says she just has different beliefs than them. It becomes clear that Angela and Brian are not getting along with one another as well and that happiness from the beginning is melting away. Angela starts to suspect Katie is planning to keep the baby and disappear and accuses her of killing Linda because she knew she was going to be the surrogate and that Katie wanted it for herself.Katie fakes early labor pains. Brian is furious with Angela and thinks she taking medicines and losing her mind. The next morning while Brian is at work, and Katie is sleeping in the girl's bed in the main house, Angela sneaks into the guest house to snoop and discovers Katie's secrets, confirming that she is mentally ill and she was a previous donor.Angela gets a DNA test for the girls but realizes that Katie has drugged her and rushes home where she confronts Katie. Katie admits everything and then stabs herself in the stomach, possibly harming the baby, telling Angela she will tell everyone that she tried to kill her. Katie is taken to the ER and an emergency C-section takes place. A healthy baby boy named Gabriel is born. But what is later revealed is that the DNA test came back and showed that Maddie and Cora were related to each other and to Katie, meaning Angela had told the truth. It is also revealed that she killed Maddie's parents before abducting her and claiming her as her own. In the end, Katie is locked up and Angela, Brian, Maddie, Cora and baby Gabriel are a happy family.