Cujo is a novel by bestselling horror author Stephen King. Cujo is a family dog who is bitten by a rabid bat. Cujo's illness goes unnoticed by his owners, who are embroiled in their own drama, allowing the illness to progress to the point that Cujo turns from a gentle, loving dog into a horrifying killer. At the height of Cujo's illness, a woman brings her child to Cujo's home, which happens to also be the location of a remote car repair shop, and where she finds herself stranded in a no longer functioning vehicle. Cujo is a fairy tale for adults, one with so many unthinkable but altogether possible coincidences, which will give any parent enough nightmares to last a lifetime.

Tad Trenton thinks there is a monster in his closet. Despite the reassuring words of his parents, Tad is certain that a monster with red eyes is hiding in the closet. The only thing that allows Tad to sleep peacefully through the night are the Monster Words, a litany that Tad's father has created to keep monsters away.

Donna and Vic Trenton, Tad's parents, have not been getting along. Vic is afraid his wife has a lover. In truth, Donna has been having an affair with a local man, Steve Kemp, but she has recently ended it. Unfortunately, Donna's lover is not happy with the end of their relationship and he has written a note for Vic making it clear that there was an affair.

Vic and his partner, Roger, are struggling to keep their advertising business, AdWorx, from going under. Their largest client, Sharp Cereals, has recently had a public debacle when a bad batch of food dye in their newest cereal caused a number of children to appear to be bleeding internally. This debacle is made worse by the fact that AdWorx's most popular commercial for Sharp Cereal is an ad in which a man, the Cereal Professor, tells viewers that there is nothing wrong with the cereal. Now, in the midst of this scandal, Vic and Roger think they are going to lose this client and cause them to lose the majority of their income. Roger has decided they should go on a two week long trip to gather the information they need to create a new campaign that will repair the damage. This trip happens to come four days after Vic learns of his wife's affair.

Donna's blue Pinto is having carburetor trouble. Once, a year ago, Vic took his car to Joe Camber, a local mechanic with a place in the more rural area of Castle Rock, and was happy with his service. Vic wants Donna to take the Pinto to Camber, but forgets to make the arrangements before he leaves. Donna is left to decide if she should drive the car to the next town and hope it does not seize up before she reaches it, or take it to Camber's even though she cannot reach him by phone.

Donna cannot reach Joe Camber by phone because he is dead. Camber's dog was bitten by a rabid bat several days before and has developed rabies without anyone in the Camber family noticing. The family has had its own drama. Charity Camber, Joe's wife, won a small sum of money in the lottery and has used it to convince Joe to allow her to take her son to visit her sister in Connecticut. Joe agrees because the trip would mean he could go to Boston with his drinking buddy, Gary Pervier, to see a baseball game and visit some places of ill repute. Joe drives his wife and son to the bus station, then returns home to finish up his work only to find himself worrying about the dog. Joe goes down to Gary's to ask his opinion on who he might get to watch the dog. However, Joe finds Gary dead. Joe goes into the kitchen to call the police, unaware that the dog has been sleeping in the basement. The dog finds Joe and kills him, too.

Donna plans to leave her son with a babysitter and take the car to Camber's for repairs. However, Tad has a nightmare and talks his mother into allowing him to go with her. Donna and Tad take a few snacks and drive the car to Camber's. The car makes it, without incident, until they reach Town Road No. 3 where the Camber garage is located. Donna nurses the car up the road and is pulling into Camber's driveway when the car quits. Donna gets out to look for Joe when Cujo appears. Donna recognizes immediately that Cujo is rabid. Donna jumps back into the car and tries to start it, but the car will not start.

Donna reassures Tad and tells him they should wait for the car's engine to cool before they try to go again. However, when Donna tries the car again, it starts for only a moment and then quits. Donna tries again and again to start the car, but the battery begins to fail. Donna then honks the horn repetitively, hoping to get the attention of the Camber's one neighbor, unaware that that neighbor is Gary Pervier and that he is dead.

Out of options, Donna and Tad wait out the night in the car. The following day, Donna waits for the mailman to come, thinking that he can rescue them when he comes with the Camber's mail. However, Joe Camber called the post office and requested his mail be held for a few days. Toward the end of the day, when it is obvious there will be no one coming to their aid, Donna focuses on the porch door. Donna believes she can make it to the door before the dog, but she is afraid it will be locked and that the dog will get her before she can get inside. Donna is frightened what will happen to Tad in that circumstance.

Toward the end of the second day, Donna decides she has to try to reach the porch door because Tad is becoming ill from dehydration. While Tad is sleeping peacefully, Donna opens the door, testing it. The door is badly dented from Cujo ramming into it repetitively, so Donna is concerned that it might not shut again. Donna closes the door, testing it. When Cujo does not appear in response to the noise, Donna becomes confident that he has wandered off or died. Donna steps out of the car and tosses some stones over the front of the car where she last saw Cujo. When nothing happens, Donna starts around the car. Cujo suddenly appears from the front of the car and charges Donna. Donna manages to get back in the car, but the dog follows. Cujo bites at Donna's belly and leg, leaving massive wounds in Donna's thigh before she is able to get it out of the car.

Vic tries to call home that night and is mildly concerned when Donna does not answer. Vic continues to call one on the hour until midnight. Now Vic is frightened that something bad has happened. Vic calls the police and has them check the house. The police call back and tell Vic that someone has broken in and vandalized the house. Vic rushes home, convinced that Donna's lover, Kemp, has kidnapped her and their son. Although the DA's investigator is confused by Donna's missing car, he also focuses on Kemp. While the DA's investigator and the state police officer go to talk to Kemp after his arrest, the sheriff goes out to check if Donna left her car at Camber's as Vic suggested she might have. When the sheriff arrives, he not only finds Donna and Tad, but he also finds Cujo. Cujo attacks the sheriff, killing him.

Tad is having convulsions and Donna is frightened for his life. Donna climbs out of the car and grabs a broken bat that has been lying on the side of the driveway. Donna attacks Cujo, beating him until the dog finally stops attacking her. Donna then continues to beat the dog. In the meantime, Vic has driven up to Camber's in search of his wife and he drives into the yard as Donna continues to beat the dead dog. Vic pulls Tad out of the car, only to find that he has been dead for some time. Donna cannot believe what Vic has told her and she performs CPR on Tad's body. When the police come, Donna has to be sedated and forcefully removed from her child's body.

Vic and Roger retain the Sharp account, but Vic cannot concentrate on work. Donna survives her injuries, but spends more than a month in the hospital. When she comes home, Donna is overwhelmed with guilt. Vic, too, suffers guilt, wondering what might have happened had he gotten to Camber's sooner. Charity and Brett Camber return to Castle Rock and decide to keep their farm despite a lack of a sizeable income. Six months after Cujo's death, Charity gives her son a new puppy.