Adrienne Saunders is happily married to her art dealer husband, Jack Jones [Judd Nelson], and they have a daughter named Mary. After the local museum curator is mysteriously murdered, Jack falls under suspicion of selling forged treasures to the museum. He has to travel to Boston on a work-related trip, but Adrienne hears from a friend that she thought she saw him in town. She confronts him, but he denies being in town. As pressure mounts on Jack over a forged relic, Adrienne receives word from the police that Jack died in a car accident. In trying to wrap up Jack's affairs, Adrienne begins to suspect that her husband was not who he claimed to be. When she sees a high school yearbook picture of her husband attributed to a man named Frank Sullivan, she realizes that she has been deceived.
She tracks down Jack's cousin, Evelyn, who confirms that Frank and Jack were inseparable in high school. After Jack died, she never saw Frank again. Evelyn explains that Frank's father was an alcoholic and that his mother worked as a toll booth operator. She directs Adrienne to Frank's mother, who lives in a rundown Brooklyn apartment. Frank's mother, Rosalie, bitterly receives the news of her granddaughter, telling Adrienne that Frank was always selfish and never looked in on her.
A stalker lurks at Adrienne's loft. He comes in to Adrienne's bed while she is asleep and caresses her. He watches Mary, who is spooked by the man in her room at night. One day, as the housekeeper finishes her chores, she surprises the stalker. He leaves her almost dead in the bathroom and ransacks the apartment.
At work, Adrienne gets an urgent message from Mrs. Sullivan and rushes to her apartment. When she arrives, the door is open, and Mrs. Sullivan is nowhere to be found. "Jack" appears, and Adrienne slaps him for his cruelty. Jack explains that when his friend died, he was distraught and fell into Jack's identity during the mourning process. He reveals that a man named Dan Sherman is blackmailing him. Jack faked his death to escape, knowing that he would have to give up his life with Adrienne and Mary. He tells her that Sherman is insistent on having an Egyptian necklace in their apartment, and he asks Adrienne to look for it. As she leaves the apartment, Jack watches her from the window beside the body of his murdered mother.
During her search for the necklace, Adrienne discovers a Parks Department photo ID. It bears her husband's picture and the name Daniel Sherman. She tracks down an address and pays a surprise visit to the house. A pregnant Mrs. Sherman is on the phone and lets her in, thinking she is with a moving company. Adrienne looks around the house and sees wedding pictures of her husband with Mrs. Sherman. In a photo album, she sees a picture of Mary, who Mrs. Sherman says is her husband's dead sister. The person on the phone is Jack, who asks her to give the phone to Adrienne.
He congratulates Adrienne on tracking down his new life and reveals that he has kidnapped Mary. Mary traded the necklace to another girl, and Jack instructs Adrienne to retrieve it and meet him at their loft to exchange Mary for the necklace. At the loft, Adrienne asks to see Mary, and Jack explains that she is downstairs playing in the car. When Adrienne tries to go see her, Jack pins her against a wall and demands the necklace first. Adrienne stabs him and flees. After a long chase throughout a construction area, Jack corners her in the freight elevator. He reveals his true nature as always doing > what comes next < no matter how difficult it is. Adrienne lures Jack into the elevator shaft, where he falls to his death; Adrienne had been holding on to an unseen elevator cable to give the illusion of being in the elevator car. Later, Adrienne and Mary pack up to move out of the loft and start a new life somewhere else.
