Obsessed

Derek Charles (Idris Elba) is the Executive Vice President of Gage Bendix, and works under Joe Gage (Bruce McGill). He and his wife Sharon (Beyoncé Knowles) have an infant son, Kyle (Nathan and Nicolas Myers). At work, Derek briefly flirts with temp Lisa Sheridan (Ali Larter), who later attempts to seduce Derek. Derek repeatedly rejects her, however Lisa continues to advance on Derek, and attempts to make out with him at the no-spouse Christmas party and flashes him in his car. Intending to report Lisa to his firm's human resource management, Derek discovers that she has quit her job.

Weeks later Derek and his workmates visit a resort for a conference, where he spots Lisa. He confronts Lisa, who spikes Derek's drink. Incapacitated, Derek is somewhat helpless when Lisa follows him into his hotel room and kisses Derek. He confronts Lisa again the following day, and hours later discovers her lying in his bed after attempting suicide through drug overdose. After repeated attempts to reach Derek on his phone Sharon finds Derek at the hospital, and suspects that Derek and Lisa had an affair, as Lisa claims. Detective Monica Reese (Christine Lahti) interrogates Derek and becomes skeptical of Lisa's claims, and informs Derek of her belief in him. Sharon kicks Derek out of their house, and he moves into a separate apartment.

Three months later, while Derek and Sharon are out reconciling their relationship, Lisa breaks in their house and tricks the babysitter Samantha (Scout Taylor-Compton) into letting her in under the pretense of being one of Sharon's friends, and flees with Kyle. When Derek and Sharon return to their house, Samantha informs them that their friend "Kate" came by to give Kyle a present. In panic, Derek and Sharon rush to Kyle's room and find out that Kyle has been abducted. Derek goes to his car with the intent to pursue Lisa, and finds Kyle sitting safely in the back seat. Derek and Sharon immediately take Kyle to the hospital for a check-up. When they return home, Derek and Sharon find that Lisa has trashed their bedroom and removed Sharon's face from their family portrait, after which Sharon leaves a threatening voice mail on Lisa's phone. Derek and Sharon set up a home alarm system, while Lisa spies on the house from outside.

Days later, Lisa learns that Derek and Sharon will be away from town, with Sharon leaving that afternoon and Derek the next morning. While Sharon is on her way to pick up Kyle, she realizes that she forgot to set the alarm system and returns home. Meanwhile, Lisa breaks into their house and decorates the master bedroom seductively. While setting the alarm, Sharon hears Lisa in the bedroom. Sharon tells Lisa that she is calling the police, but Lisa stops her and the two engage in a fistfight. Derek calls his house and Lisa answers; Derek calls Detective Reese and immediately leaves his office.

Lisa runs to the attic, and Sharon pursues her. Sharon leads Lisa to a weak spot in the attic floor, which she falls through. Sharon reaches out and takes Lisa's hand to lift her up, 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. Lisa falls onto a chandelier, stopping her fall, but lets go and falls onto the glass table below. Lisa opens her eyes, only to have a chandelier fall and kill her. Derek and Detective Reese arrive as Sharon comes out of the front door. The film ends when Derek and Sharon embracing while Detective Reese enters their house to investigate Lisa's actions.