Following a painful divorce from his wife Nickie, Robert Forester leaves New York and moves to a small Pennsylvania town, where he develops an obsession with the seemingly happy, 23-year old Jenny Thierolf. As a way of easing his own discontent with life, he begins spying on her through her kitchen window, and is surprised when she invites him into her house after spotting him one night.
Jenny sees their chance meeting as an act of fate, and breaks off the engagement to her hot-tempered fiancé Greg Wyncoop. During the next weeks, Jenny in turn goes after Robert, contacting him at his home and the company he works at. Robert is offered a promotion at work (which will require him to move to another city), and he hopes this will put an end to Jenny’s advances, which he is feeling increasingly uneasy about.
One night, Greg starts a fight with Robert, which ends in Greg being knocked unconscious and left on a river bank by Robert. Soon afterwards, Greg is reported missing, with Robert becoming a suspect for the police. Also, Robert’s ex-wife Nickie tells the police that Robert once threatened her with a weapon. After a newspaper article on the case appears, Robert’s promotion is withdrawn. A badly decomposed body is found in the river which the police believe to be Greg’s, but the identification proves to be difficult. The fragile relationship between Robert and Jenny deteriorates quickly, and finally Jenny commits suicide after coming to the conclusion that Robert’s appearance in her life symbolizes her death.
Nickies new husband Ralph informs Robert that Greg’s disappearance is actually a plot of Greg and Nickie against him. Later, Robert is shot at by Greg (severely wounding another person instead). Greg is arrested by the police but released. In a final confrontation between Robert, Greg and Nickie, Nickie is accidentally killed when Greg tries to knife Robert. Again, Robert is a suspect in an apparent crime scene.