Daniel "Dan" Gallagher is a successful and married lawyer from Manhattan. He meets Alexandra "Alex" Forrest, an editor for a publishing company. While his wife, Beth, and daughter, Ellen, are out of town for the weekend, Dan has an affair with Alex. Initially, it seems that both understand it to be just a fling, but Alex begins to cling to him.
Dan reluctantly spends the following day with Alex at her request. When he tries to leave again, she cuts her wrists in a move to manipulate him into staying and saving her. Dan helps her, stays overnight to ensure she is all right, and leaves in the morning.
Alex shows up at his office to apologize for her behavior and invites him to a performance of Madame Butterfly. He declines politely but firmly, but she continues to call at his office until he informs his secretary that he will no longer take her calls.
Alex insists that Dan meet with her and informs him that she is pregnant, arguing that he must take responsibility. After Dan changes his phone number, Alex meets Beth, who has advertised selling their apartment. That night, Dan goes to Alex's apartment to confront her, and they get into a scuffle. One of the movie's most famous lines, she declares, "I'm not going to be ignored, Dan."
Dan relocates his family to Bedford, but this does not dissuade Alex. She has a tape recording of herself delivered to him, which is full of verbal abuse. She stalks him, pours acid on his car, and follows him home later that evening from a rental car dealer. The sight of his happy family through their window makes her vomit.
Alex's obsession escalates when Dan approaches the police to file a restraining order, claiming it is "for a client." The lieutenant informs him he cannot violate Alex's rights without probable cause, and the "client" must own up to his adultery.
The Gallaghers return home one day soon after, and Beth finds Ellen's pet rabbit killed, and boiling in a pot on their stove. Dan knows it was Alex, and following this, he confesses the affair and Alex's pregnancy to Beth. Enraged, Beth orders him to leave the house. Prior to departing, Dan calls Alex to say his wife knows about the affair. Beth takes the phone and says she will kill Alex if she persists.
Soon thereafter, Alex somehow picks Ellen up from her school and kidnaps her, taking her to an amusement park. Beth drives around frantically looking for her, and gets into an accident, requiring hospitalization. Alex returns Ellen home unharmed later that day.
After visiting Beth in the hospital, Dan forcibly enters Alex's apartment and attempts to strangle her, but stops short of killing her. She grabs a kitchen knife and lunges at him, but he disarms her and departs. The police search for Alex after Dan reports the kidnapping, but can't find her. Beth attempts to forgive Dan and the couple returns home.
That evening, Dan is downstairs making tea for Beth. Beth runs a bath, but before she can get in, Alex appears with a knife and explains that Beth is obstructing her from having Dan. She attacks her, and hearing the commotion upstairs, Dan rushes in, appearing to drown her. Alex suddenly emerges from the water, brandishing the knife. Beth quickly returns with a gun and shoots Alex, who is seen bleeding from her chest, with a look of shock on her face, before dying in the tub. Dan completes his statement to the police and joins Beth in the living room, with a picture of their family in the foreground.
