Dan Gallagher is a successful, handsome and happily married attorney living in Manhattan with his wife, Beth, and their daughter Ellen. Through his business he meets Alex Forrest, who works as an Editor for a publishing company. The two have an obvious frisson and one weekend, whilst Beth and Ellen are out of town, Dan sleeps with Alex. Dan believed it was a mutually-understood one-night-stand, just a fling, but Alex sees it as an affair, at the very least, and begins to show signs of being clingy.
Unplanned, Dan stays with Alex for a second evening. This time when he tries to leave Alex slashes her wrists and attempts suicide. Dangelos her to bandage them up then leaves. He thinks the affair is forgotten and to him, it is, but to Alex it takes on a far greater significance and she begins to turn up at various places just to see Dan. She waits at his office one day to apogize for her suicide attempt,inviting him to a performance of the opera "Madame Butterfly" but Dan politely turns her down. Alex continues to try to contact him, barraging him at work with telephone calls until he tells his secretary that he will no longer take the calls at all. Alex is not to be rebuffed and telephones him at home at all hours of the day and night, subsequently confronting him to tell him that she is pregnant and plans to keep the baby. Dan wants nothing to do with her but Alex argues that he must take responsibility for their child. She also shows up at his apartment, which he and Beth have put on the market, where she meets Beth and pretends to be an interested prospective buyer for the property. Angered, Dan goes to her apartment later that night to confront her and a violent scuffle results. Alex tells him that she will not be ignored.
Dan moves his family to Bedford but this does not deter Alex; in fact it appears to have the opposite effect. She is a skilled stalker and has a tape recording delivered to him, filled with verbal abuse. She stalks him in a parking garage and pours acid on his car then follows him home to spy on him from the bushes in the yard. Seeing Dan with Beth and Ellen literally makes her sick to her stomach. Her constantly-escalating obsession prompts Dan to approach the police to apply for a restraining order which he claims is for a client. The Lieutenant will not violate her rights without probable cause and tells Dan that his "client" should own up to his adulterous behavior.
Alex waits until the Gallaghers are not home then enters the house. She kills Ellen's beloved pet rabbit and puts it on the stove to boil. This prompts Dan to own up to his affair and he tells Berh that Alex is pregnant. Beth is understandably hurt and furious and demands that Dan leave. Before he does so, he calls Alex to tell her Beth knows about the affair. Beth takes the telephone from him and tells Alex that she will kill her if she persists in this behavior.
Unbeknownst to Dan and Beth, Alex picks Ellen up from school and takes her to an amusement park, taking her on a roller-coaster and buying her ice cream. Beth panics when she realizes that she does not know where her daughter is. She drives around frantically searching for her and rear-ends another car an an intersection.she is injured in the altercation and is hospitalized. Alex later takes Ellen home and aske her for a kiss on the cheek goodbye. Following Beth's release from the hospital she chooses to forgive Dan and they return home together.
Dan barges into Alex's apartment, so enraged that he attacks her, choking her and almost strangling her to death. He stops himself but as he does she grabs a kitchen knife and lunges at him with it. He overpowers her, putting the knife down on the kitchen counter and leaving. Alex watches him leave, leaning against the counter with an hysterical smile on her face.
Dan approaches the police for a second time to ask them to arrest her so they start searching for her.
Beth is at home preparing a bath for herself when Alex appears suddenly again holding the kitchen knife. She starts to explain to Beth her feelings of resentment towards her, fidgeting with the knife nervously, causing her leg to start bleeding before attacking Beth. Dan hears screaming and rushes to the bathroom to defend Beth. He wrestles Alex into the bathtub and seems to have drowned her but she emerges suddenly from the water, bolt upright, swinging the knife. Beth, who left the room to look for Dan's gun, returns and shoots Slex on the chest,immediately killing her. The final scene shows police cars outside the Gallagher's house. As Dan finishes talking to the police,he walks inside the house where Beth is waiting for him. As they embrace and proceed upstairs the camera focuses on a family photograph of both Dan and Beth with Ellen.