Thursday the 12th unfolds as Waldemar (Jeffrey Tambor) and his wife Yolanda (Nancy Lee Andrews), both vampires hailing from Transylvania, are on the brink of finalizing a deal for a home in Erie, Pennsylvania. However, their plans take an unexpected turn when they discover that the property has been inherited by a family of four who is set to move in while the couple discusses the transaction with their realtor.
John (Richard Benjamin) and Mary (Paula Prentiss) are attending the reading of a will, where they learn that John's recently deceased uncle has left them the house. Upon arrival, the couple is enamored with the property's charm, despite its evident need for repairs and the ominous rumors of a curse. Their children, 16-year-old Debbie (Kari Michaelsen) and 12-year-old Billy (Kevin Brando), do not share their enthusiasm. Not even Rover, the family dog, is eager to step inside. "You'll be amazed at what some nice curtains will do for this house," Mary reassures them, attempting to lift spirits.
That evening, while trying to shake off boredom in front of the television—which seems only to air reruns of The Twilight Zone—Billy decides to explore the library. He stumbles upon The Book of Evil, which piques his interest. Despite a dire warning inscribed within, "Whoever shall open this book releases into the world all the evil herein," he can't resist flipping through its pages. Each turn reveals illustrations of monsters that inexplicably vanish. Later that night, Billy is startled awake by a monster in his room, while Mary is unexpectedly bitten by Waldemar, and the family is kept up by chaotic "owls" in the yard. From a distance, Waldemar and Yolanda watch in horror as they realize someone has disturbed THE BOOK.
On Friday the 13th, as John and Mary venture out for curtains, Billy uncovers that his mother has tidied his room and inadvertently relocated THE BOOK. Debbie, meanwhile, finds herself terrified in the bath when a mysterious fin appears, revealing a gill-man creature that pursues her, ultimately targeting a hapless delivery man instead. Later, Mary faces a swarm of bats in the attic, reminiscent of The Birds, prompting the family to call upon an exterminator, Mr. Van Helsing (Severn Darden). He confidently assures them he will remedy the bat situation, stating, "You don't expect to get rid of bats overnight!" Over dinner, Billy reveals that he has opened and misplaced THE BOOK, prompting Van Helsing to caution that the world is in peril if it isn't recovered.
As Saturday the 14th arrives, John and Mary are preparing for a gathering with family and friends. However, when Billy goes missing, the guests disperse in search of him. Just before midnight strikes, Waldemar and Yolanda arrive, bringing Billy and THE BOOK. As monstrous beings close in, Van Helsing urges Billy to relinquish THE BOOK, but to no avail. Billy's confrontation reveals he recognizes Van Helsing from a picture in the book, realizing he, too, is one of the monsters. A tense standoff ensues between Van Helsing and Waldemar, with the two exchanging grimaces. In a sudden twist, Van Helsing snatches THE BOOK, vanishing along with all creatures. Instead of freeing the beasts, the destruction of THE BOOK eradicates them all.
In the concluding scene on Sunday the 15th, John, Mary, Debbie, and Billy make their escape from the haunted house, while Waldemar and Yolanda settle into their new abode.
