Vic and Donna Trenton struggle to repair their crumbling marriage. Their young son, Tad, befriends a hulking, lovable, 200-pound St. Bernard named Cujo. With Vic away on business, Donna and Tad take their rundown car to be fixed at the remote farm of their mechanic. As their Pinto dies, Cujo appears. But the once docile dog has undergone a hideous transformation, and becomes a slavering, demonic, implacable killer possessed by almost supernatural strength, not to mention cunning.