The novel is seen through the eyes of Harold, the Monroe family's dog, and takes place in a comfortable suburban home.
The Monroe family, which includes Mr. and Mrs. Monroe and their two sons, Toby and Pete, finds a rabbit at a movie theater during a screening of Dracula. The family brings the rabbit home and names it Bunnicula. Harold, the dog, and Chester, the cat, already part of the Monroe family, have mixed reactions to the new pet. While Harold is more welcoming, Chester becomes suspicious of the rabbit.
Chester, a well-read cat with a flair for drama, becomes convinced that Bunnicula is a vampire. Bunnicula's odd sleeping patterns arouse his suspicions—he sleeps all day and is awake at night, and his unusual bunny features like pointed fangs and strange dietary habits. The family begins to find vegetables drained of their color and juice, which Chester attributes to Bunnicula's vampiric tendencies.
Concerned for the family's safety, Chester goes to great lengths to prove his theory and protect the family from Bunnicula. He stages interventions, shares stories about vampire lore, and even attempts to put a steak on Bunnicula to ward off his alleged vampire powers. Though dubious about Chester's assumptions, Harold gets dragged along in these plans.
As the story progresses, Chester's attempts become more drastic, culminating in his trying to lock Bunnicula in a cage without food to force him into revealing his vampiric nature. Harold, realizing Chester's plan might harm Bunnicula, intervenes. He breaks into the refrigerator and brings Bunnicula vegetables to keep him from starving.
Ultimately, the family finds the weak Bunnicula and rushes him to the vet, who diagnoses him with malnutrition. The family decides to take better care of Bunnicula, ensuring he's well-fed and looked after. Chester is discouraged from harassing Bunnicula, and the family returns to its peaceful life. Chester, however, remains somewhat suspicious of Bunnicula, leaving room for more potential adventures.
