Ellen Carson is a real estate agent who inadvertently cuts off a delivery truck driver while changing lanes on the freeway to hurry home. The truck driver turns out to be a disturbed man named Eddie Madden, who proceeds to chase after Ellen in an effort to run her off the road. Ellen in fear calls the 1-800 number on the back of his truck and lodges a complaint, which causes Eddie to lose his job, and he (being a grieving husband and father who earlier lost his family to a car accident) sets out to destroy Ellen's family and soon becomes fixated on Ellen and her teenage stepdaughter Cynthia and plots to have them as replacement family, by removing the head of the house, Ellen's husband and Cynthia's father Jim Carson.

Eddie Madden, un livreur, perd sa femme et sa fille dans un accident de la route. Six mois plus tard, le veuf est au volant de sa camionnette de livraison quand une automobiliste lui coupe la route. Fou de rage, il la prend en chasse. Elle parvient à le semer, non sans avoir noté le numéro de l'entreprise figurant sur sa camionnette, et va porter plainte. Eddie est licencié. Avant de quitter son poste, il relève le nom de la responsable de son malheur. Il s'agit d'Ellen Carson, agent immobilier. La jeune femme mène une vie sans histoires avec Jim, un architecte, et sa belle-fille, Cynthia

Mentre si trova in viaggio di lavoro alla guida della propria automobile, la giovane Ellen Carson accosta casualmente la vettura di Eddie, un uomo il cui equilibrio psichico è stato seriamente compromesso da una serie di drammatiche vicissitudini familiari. Per Ellen e i suoi cari si tratta dell'inizio di un incubo...