Obsessed

Derek Charles (Idris Elba) is the Executive Vice President of Gage Bendix and has received a recent promotion from his boss (Bruce McGill). He is happily married to Sharon Charles (Beyoncé Knowles), who is pursuing her business degree. They have a son, Kyle. However, a new temporary worker, Lisa Sheridan (Ali Larter) begins to unnerve him. Derek's best friend and co-worker, Ben (Jerry O'Connell), reminds him to be careful because he thinks she has her eyes on him. Lisa eventually attempts to seduce him, but he repeatedly rejects her advances. Lisa becomes obsessed and begins to manipulate Derek and Sharon's relationship, eventually becoming violent. Lisa's actions become erratic, and she fakes a suicide in Derek's hotel room while he is on a business trip, leaving him no choice but to send her to the hospital. Meanwhile, Sharon is trying to reach Derek, but she can't get in touch with him, so she calls Ben. Ben tries to cover for Derek, but he just comes out and tells her that he should tell her the truth himself. This prompts Sharon to go to the hospital where Derek is. Detective Monica Reese (Christine Lahti) interrogates Derek about his "relationship" with Lisa. Both Detective Reese and Sharon refuse to believe that he didn't sleep with Lisa. When Derek and Sharon go home, Sharon kicks him out of the house, and they separate for three months. Derek moves into his own place, but he is too lonely without his family. Sharon still lets him visit and play with Kyle, but she rudely makes him leave immediately when he's done.

Within the three months, Detective Reese interrogates Lisa, who lies about her relationship with Derek. Detective Reese discovers that Derek was telling the truth and that Lisa is insane. Reese informs Derek that she believes him now, and Lisa has moved to San Francisco. As an attempt to win Sharon back, Derek asks her to go out to dinner with him for his birthday. After a long talk, Sharon forgives him and gives him a gift: his key to the house, which she had taken from him when he moved out. While they are out, Lisa goes to their home and convinces the babysitter Samantha (Scout Taylor-Compton) to let her in, eventually escaping with Kyle while Samantha is distracted. When Derek and Sharon return, Samantha informs them that Lisa had been there, and they find that Kyle is missing. Derek goes to the car, intending to pursue Lisa, and finds Kyle in the backseat with lipstick from Lisa on his forehead. They immediately take Kyle to the hospital to make sure he is okay. Detective Reese meets up with them there. Sharon warns her that she better do something about Lisa, or she'll do it herself. Reese assures them that she'll handle it. They take Kyle home, where they have a hard time putting the upset baby to bed. Afterwards, they try to go to bed, only to see that Lisa has trashed their bedroom and removed Sharon's face from all the family portraits. The next morning, Sharon leaves an angry, threatening voicemail on Lisa's phone. They set up a home alarm system, while Lisa secretly monitors the house from outside.

Later on, Patrick (Matthew Humphreys), Derek's assistant, receives a call from Lisa. He reveals that Sharon and Derek will be visiting her mother's house, with Sharon leaving that afternoon and Derek first thing the next morning. While Sharon is on the way to her sister's to pick up Kyle before she goes to her mothers, she calls Derek, who asks her if she set the alarm. She realizes that she forgot and goes back to set it. Meanwhile, Lisa has broken into the house with a bottle of champagne. While setting the alarm, Sharon hears Lisa pop the champagne bottle open. Sharon finds Lisa wearing one of Derek's t-shirts lying in her bed. Sharon tells Lisa that she is calling the police, but Lisa tries to stop her. Sharon pushes Lisa down, but Lisa gets up and shoves Sharon onto the dresser, knocking her down, and begins kicking her. Lisa grabs a lamp and tries to hit Sharon with it, but Sharon escapes to the bathroom.

Derek calls the house, and it's Lisa who answers. Sharon struggles with Lisa to get the phone, headbutting her and knocking her down. Sharon tells Derek she's going to have to call him back and hangs up. Derek calls Detective Reese and leaves his office. While fighting, Sharon reminds Lisa of everything she has done. Lisa escapes and runs to the attic. Sharon pursues Lisa to the attic, where she is attacked. Sharon, remembering when Derek showed her a weak spot in the floor, leads Lisa to it. Lisa falls through the floor and hangs onto the edge. Sharon reaches out and grabs her hand to prevent her from falling, but Lisa pulls Sharon down with her instead of accepting her help. Seeing that the floor is starting to buckle, Sharon pries Lisa off of her arm, and she falls to the ground. Lisa grabs a chandelier, stopping her fall, but lets go and falls onto the table below. Sharon sighs in regret as she sees this but then sees Lisa open her eyes. Lisa begins to rise up when the broken chandelier falls on Lisa and seemingly kills her. Derek and Detective Reese arrive as Sharon comes out of the front door. As the film ends, Sharon and Derek embrace each other.