The story starts with a little rabbit peeping its head out of a hole in the ground. It bounds through the forest and emerges in a meadow where it sits up on its hind end to look around. Then a Saint Bernard (Cujo) appears and chases it around, eventually into a cavernous hole in the ground. Cujo sticks his head into the hole and barks, disturbing the bats who have roosted in there for the day. They fly around and screech and one lands on Cujo’s snout and gnaws on him for awhile.
Now we go to the big Trenton house. Tad, the son, has a problem with his closet door popping open. He closes it, then shuts off the light and makes a mad dash across the room for the bed. He stares at his closet door, it opens, and he screams. Vic and Donna come in to comfort him. Tad insists he saw the monster. Mom and Dad say there’s no such thing as real monsters. After they go back to bed, Tad says, “Except in my closet. Please, please, please.” The next morning, everything that isn’t nailed down is piled in front of Tad’s closet door. Steve Kemp comes in and mooches off the Trentons. On the television is an advertisement for Sharp’s cereal, a professor who says, “Nothing wrong here!” Vic works for an advertising firm, and the professor is one of his creations.
Vic and Kemp play tennis, and Kemp loses. Vic says, “You aren’t getting tired of this, are ya?” Kemp replies, “What, are you kidding me? Getting my ass handed to me every week? No, I love it. I’m a masochist.” “Well, whatever turns you on,” says Vic. Cut to Kemp playing his trombone (literally, not figuratively) in bed with Donna Trenton. Donna puts on her underwear and goes home. At dinner that night, Vic suggests they have another kid because they’ve run out of stuff to talk about.
Vic takes his car into the shop with some complaint about a front wheel. He wants it fixed while he waits, which the mechanic won’t do. The mailman suggests he take it out to Joe Camber. Vic does, and he takes Donna and Tad with him. Donna wanders in front of the house, where Charity Camber is plucking a dead chicken. Then she sees Cujo trotting into the barnyard and runs over to snatch up Tad. The Cambers’ son Brett says it’s all right, and eventually Donna gets a grip and lets Tad down to pet the dog. That night Vic makes the rounds in Tad’s room, saying some strange incantation to ward off monsters. It seems to make Tad feel better. The next day the news says that thousands of people reported internal hemorrhaging after eating Sharp’s cereal. The cereal was recalled, but it turned out kids were only “peeing and puking” red dye. Vic is facing a crisis at work, though.
Donna goes to see Kemp to break off their affair. He acts resigned, but then chases after her. Vic drives by and sees Kemp grabbing her by the car. He goes back, but they’re gone. Donna picks up Tad from camp. The car has some serious problems on the way home. Vic is home early and asks what she did that day; she claims groceries, errands, the usual.
Joe comes flying up the drive and grinds to a halt in his noisy truck. Cujo stalks off. Joe finds an engine hoist in the barn and storms into the house, cussing at Charity because they can’t afford it. She tells him she won $5,000 in the lottery, and she wants to take their son to visit her sister for a week. Joe is visiting with his friend Gary; they’re making a lot of noise and Cujo is lying on the floor making distressed noises. Joe has decided to take off for a week of “broads, booze, and baseball” and wants Gary to join him.
Vic picks up Tad from camp early. Back at home, Donna’s gets a surprise visit from Kemp. He wants her back, says “I miss touching you,” then proceeds to try. He slides his hand up her dress and kisses her and when she finally fends him off, he shoves her, which spills some milk and brings Vic and Tad into the kitchen. Kemp leaves. Vic looks at Donna and says, “Yes or no,” and she says, “Yes.” Vic tries to fix the Pinto and can’t. He’s going to be out of town for ten days to try to salvage the Sharp’s account. He promises Tad to write down the monster words so that Mommy can say them. Tad sleeps soundly with the monster words posted on his closet door.
The next morning is very foggy at the Camber house. Brett hears some moaning-yelping-whining noises and goes out to call Cujo. After awhile he hears some vicious growling, so he turns to run home and finds it’s Cujo behind him. Brett tries to soothe him, and Cujo stops growling and walks off into the fog.
Vic forgot to take the Pinto to Joe’s, so Donna will have to do it. As he drives off, Donna runs after him. She tells him that the affair is over, but she can’t make like it never happened. He says he can’t either and drives off.
Brett’s loading the car for the trip. He tells his mother, “I’m worried about Cujo. I saw him this morning in the fog and he was all bloody, he was dripping foam at the mouth.” He wants to tell his father, but his mother tells him he will do no such thing.
Cujo is walking down the road. He shows up at Gary’s house and attacks him outside. Gary gets away and goes into the house, closing and locking the screen door behind himself, but leaving the large, solid wooden door wide open. He starts trying to load his gun, and Cujo starts ripping down the screen. He gets in before Gary gets the gun loaded and kills him rather messily. Meanwhile, at the Cambers’ house, Joe is calling Cujo. He finally gives up and dumps a fifty-pound bag of dog food into a tub. He goes to pick up Gary and sees the screen ripped off the door. He goes in and finds Gary’s body. He then checks most of the ground floor of the house, making his way to the phone. He picks it up and Cujo shows up. Joe takes one look at him and says, “Cujo! Oh my God, you’re rabid.” If only he hadn’t married such a cretin, he might have lived longer. He throws the table at Cujo, but it doesn’t buy him much time and the dog gets him.
Donna and Tad are singing merrily as they drive out to Joe’s place. “I think I’ll bite it,” Donna punctuates with a snap and snarl. The Pinto is ready to cack as they pull into the drive. We get a view of them from the barn. Donna gets out, but Tad’s seatbelt is stuck, so she leans in to help. Although her open door is on the barn’s side, Cujo has walked to the other side of the car and jumps up against the passenger door and sticks his head through the partially open window. Donna gets it rolled up and then closes her own door. Tad produces a lot of noise, screeching about the monster. Cujo is now on the windshield. Donna blows the car horn and he leaves. Now Tad is making a lot of hysterical noise. Donna can’t get the car started again, and Tad wails repeatedly, “I wanna go home!!!” The kid has a serious set of lungs, and Cujo lies at a distance and growls.
Later, Tad is much calmer, sitting in the back of the car drawing. Tad tells Donna to try to start the car, and she is able to get it going. Cujo comes running out at the sound. The yard is huge, but instead of making one large turn and leaving, Donna makes a short turn and then puts it in reverse. After she cusses at the dog, the car dies again. Tad starts whining fearfully, “Can he get us in here?” (We can only hope.) “Can he eat his way in here? I wish he’d die.”
Now it’s sunset, and Vic calls home, but there’s no answer. Back at the farm, Tad has to urinate, so Donna opens the door about a foot. Cujo’s lying directly in front of the car. He looks at the stream of urine hitting the ground, then the phone starts ringing in the house and he decides that’s more annoying. He runs to the house, jumps at the door, and then leaps through the window right on the last ring. Then he lies down on the porch slathering and snarling.
Sunrise. Donna wakes up to find Cujo watching her through the window and growling. Vic calls home again. Donna opens the window, “for air,” about five inches. She notices a baseball bat in the yard. She tries to start the car again without luck, then figures the mailman will notice them. Back in town, the mailman is heading out with a large delivery for Joe Camber, but one of the postal clerks reminds him of the mail hold for that week.
Phone rings at Cambers’ again. Cujo runs and looks in its direction, then turns and charges the car, bashing headfirst into the driver’s side door twice. Donna may be thinking better of leaving her window open so far with a rabid dog outside when he starts shoving his paws and face through it. Then he runs to the passenger’s side and leaps at the window, severely crunching it. He then chews off the door handle, leaps on top of the car, and mellows out a bit when the phone finally stops ringing. Tad is moaning and carrying on, and Donna’s close to smothering him trying to get him to keep quiet. Vic’s finding it difficult to concentrate on his job.
Tad is sleeping now, so Donna decides to get out of the car. She doesn’t notice that Cujo is lying practically underneath the car on the other side. She very noisy since the door is halfway bashed in; at this point we get a shot of her feet from the other side of the car. After getting the door open, she looks around from inside the car. She gets out, closes the door, and looks around. It occurs to her that the dog might have been close to the car and she just couldn’t see him from inside, so she gets on her hands and knees to look under it. Cujo’s behind her and jumps her before she can do anything. Tad wakes up to find Mommy being mauled by a rabid dog outside. Donna gets the door open and gets into the car as she normally would, i.e. climbing in and then leaning back out to pull the door closed, rather than the patented leap-and-pull that anyone with that much adrenaline in her system ought to be able to manage. Needless to say, Cujo ends up in the car with them. Tad manages to climb into the back seat. Donna beats Cujo with a Thermos container, Cujo bites her leg, and she eventually gets the door closed with the dog on the outside. Of course, Tad has been screaming the whole time at a pitch and volume that ought to be able to shatter glass. Vic wakes up from a dream, calls the house, and decides to come home. Cujo staggers toward the car. Donna bandages her leg with strips from her dress. Cujo gently climbs onto the hood of the car and lies down with his face pressed against the windshield to watch her.
Sunrise. Tad has begun to asphyxiate in his sleep. Donna tells him to wake up. Then she opens the door, apparently intending to go to the house to call an ambulance, and Cujo rushes her from the barn. She pulls Tad into her lap. First aid consists of Donna putting her fingers in his mouth, getting bitten, “Ow!”, putting her fingers in his mouth again, getting bitten again, “Ow!!” with a touch of anger this time. Then she orders him to breathe.
Kemp goes to the Trentons’ house and, finding no one home, grabs a knife from the kitchen. Vic comes home to find his wife and son missing, and various foam padding and stuffing everywhere, as well as pictures ripped up. The cops ask where the Pinto is, and they send one of the officers, Bannerman, to the Cambers’ to see if it had been dropped off. Everyone is asleep when he arrives. Cujo runs off. Bannerman notices blood on the battered car and gets out. He starts to radio back what’s going on, then hears a noise and goes to investigate instead. He gets jumped by Cujo and drops his gun. He finds a hunk of wood to beat off the dog, then is unable to climb high enough in the barn to escape him. Donna can’t get her door opened to help him, so he gets torn up a bit. She finally gets out, when it really seems he’s past saving anyway, and Cujo leaves the cop to run over and bark/snarl at her. She climbs back into the car and Tad has one of his fits. Cujo goes back to finish off Bannerman, leaving Donna to administer some more emergency treatment. This time she smacks Tad on the back a lot and shakes him like a ragdoll. He finally starts to breathe, only to start screaming, “I want my daddy!”
Vic’s cleaning up at home. The cops have rounded up Kemp, who admits to making the mess, but claims he never saw Donna or Tad. Vic asks what Bannerman found, and the investigator says, “Well, he, uh — probably following up some lead, we’ll be hearing from him anytime now.” Vic heads to Cambers in a panic (in his convertible).
Apparently Tad is having another trauma, another opportunity for Donna to practice her EMT skills by yanking his arm rather forcefully (probably dislocating it besides) and smacking his face. “I’m losing my baby!” So she gets out of the car and shambles towards the house. Cujo comes out from under the porch. Donna runs and picks up the bat, whacking him on the head no less than five times before it snaps. He then leaps at her and impales himself on the broken spike of the bat. She rolls him off of herself and picks up Bannerman’s gun. She points it at Cujo, but apparently she cannot bring herself to shoot a “dead” dog when he’s down. She has some difficulty getting back into the car to get Tad, so she ends up awkwardly smashing the rear window with the gun, something she probably learned in her gun safety course.
Donna carries Tad into the kitchen and lays him on the table. More first aid!! She splatters him with water and then, without checking for a pulse or breathing, starts pushing on his chest, then proceeds with rescue breathing. Meanwhile, Vic is racing the seven miles to the Camber farm. Donna is bent over the table, slobbering on Tad (not an exaggeration, there is very clearly a rope of saliva trailing from her mouth when she comes up for air). Cujo jumps through the window. You have to admire his creativity when, injured as he was, he much more easily could have come through the door or the broken window in the living room. Donna picks up the gun and shoots him as he recovers from his landing and turns to deal with her. Vic pulls up, and Donna carries Tad out looking a bit worse for the wear.