Outside a peaceful town in central Maine, a monster is waiting. Cujo is a two-hundred-pound Saint Bernard, the best friend Brett Camber has ever had. One day Cujo chases a rabbit into a bolt-hole – a cave inhabited by sick bats. What happens to Cujo, how he becomes a horrifying vortex inexorably drawing in all the people around him makes for one of the most heart-stopping novels Stephen King has written.
The Trentons – Vic, Donna and their son Tad – are a normal suburban family. Vic works in advertising, Donna is a housewife and Tad is a sensitive little boy who has a fear of monsters in his closet. One day, the Trenton family takes their car to the rural home of abusive mechanic Joe Camber for some repairs, where they meet Cujo – the Camber family’s easy-going St. Bernard who has a bite on his nose that Donna notices, but thinks little of it. Vic and Donna’s marriage is tested when Vic learns that Donna has been having an affair with her ex-boyfriend, Steve Kemp.
After Charity and her son, Brett, leave for a week to visit her sister, Cujo’s bite from a rabid bat, drives him mad, and he kills the abusive Joe and his alcoholic neighbor, Gary Pervier. Vic goes out of town on a business trip. Donna and Tad return to the Cambers’ house for more car repairs and Cujo attacks them. Donna and Tad take shelter in their Ford Pinto, but the alternator dies and the two are trapped. Therefore, they are forced to stay inside their car while Cujo attacks repeatedly. The hot sun makes the conditions nearly unbearable and Donna knows that she must do something before they both die from heatstroke or dehydration. Attempts at escape, however, are foiled by the mad Cujo’s repeated attacks. Donna decides that she must risk leaving the car, but Cujo bites her on the leg and forces her back inside. Eventually, the badly wounded Donna makes another desperate escape attempt but is again attacked and must return to the car. Vic arrives home to rekindle his marriage, but finds out Donna and Tad are missing. He suspects the possessive Steve Kemp of kidnapping, but then realizes his wife and son might be at the Camber’s residence. The local Sheriff comes to the house for a brief standoff, before Cujo kills him.
Meanwhile, Donna takes advantage of a momentary distraction and hits Cujo with a baseball bat until it breaks off, leaving only a jagged handle. Cujo jumps at her and is impaled by the broken bat. Donna then breaks the windshield with the sheriff’s revolver and retrieves Tad, as Cujo broke all of the door handles. As Donna revives the dehydrated and over-heated Tad in the house, a recovered Cujo breaks through the kitchen window and tries to kill them, but Donna kills him with the Sheriff’s pistol. Vic arrives and is reunited with Donna and Tad.