The story of Cujo begins in the quiet, sun-soaked town of Castle Rock, where everyday life moves at a gentle pace and danger seems far away. Donna Trenton, a young mother struggling with marital tension and personal guilt, tries to keep her family together while navigating a strained relationship with her husband, Vic. Their young son Tad is troubled by nightmares about imaginary monsters lurking in his closet, a fear that seems harmless at first but slowly foreshadows the terror that will soon invade their lives.

On the outskirts of town, Cujo, a once-gentle St. Bernard, becomes the center of a tragedy he cannot comprehend. While chasing a rabbit into a small cave, he is bitten on the nose by a rabid bat. The wound becomes infected, and Cujo’s behavior gradually transforms from friendly and loyal to unpredictable and violent. His descent into madness unfolds slowly, unnoticed by his owner, Joe Camber, who remains unaware of the danger growing right outside his door.

As Vic leaves on a business trip, Donna takes Tad to Joe Camber’s remote mechanic shop to repair their failing car. What seems like a simple errand turns into a nightmare when their car breaks down completely just as Cujo, now fully overtaken by rabies, emerges from the shadows. Trapped inside the sweltering vehicle, Donna and Tad face a horrific standoff against the massive dog, who relentlessly circles the car, snarling and attacking with terrifying persistence.

With no help in sight and the summer heat turning the car into an oven, Donna desperately tries to protect Tad as he becomes dehydrated and increasingly weak. Her attempts to escape are met with Cujo’s savage violence, and the situation grows more hopeless with each passing hour. The isolation of the rural setting heightens the dread, making every sound, every movement, feel like the beginning of the end.

Meanwhile, Vic becomes increasingly worried when he cannot reach his family, sensing that something is terribly wrong. His fear mirrors the growing desperation Donna feels as she weighs the risk of confronting the rabid dog directly. The tension rises as the film shifts between his anxious search and Donna’s fight for survival.

In a final act of courage fueled by maternal instinct, Donna forces herself to leave the car and battle Cujo directly in a brutal struggle for life. The climax is intense and emotional, showing a mother pushed beyond her limits against a creature that has become a symbol of pure, tragic terror. When the ordeal finally ends, the story leaves viewers shaken, reminded of how quickly ordinary life can unravel when fear takes hold in the most unexpected form.