A woman groans and writhes with the pain of childbirth in a bedroom from a bygone era as two elderly women – who appear to be twins – and an elderly man watch. She is then led out of the room by the elderly man.
At the Miskatonic University in Arkham, Massachusetts, Dr. Henry Armitage has just finished a lecture on the local history and the very rare and priceless book known as the Necronomicon. He gives the book to his student Nancy Wagner to return to the library. She is followed by a stranger, who later introduces himself as Wilbur Whateley. Wilbur asks to see the book, and though it is closing time and the book is reputedly the only copy in existence, Nancy allows it under the influence of his hypnotic gaze.
Wilbur’s perusal of the book is cut short by Henry, who has researched Wilbur’s family’s sordid past. His warnings about the Whateley’s go unheeded by Nancy, who decides to give Wilbur a ride back to Dunwich after he misses his bus, perhaps purposely. At a gas station on the outskirts of town, Nancy first encounters the ill-esteem in which the locals hold Wilbur.
Once back at the Whateley house, she meets Old Whateley, Wilbur’s grandfather. Wilbur disables her car, and then drugs Nancy. She decides under the influence of hypnosis and drugs to spend the weekend, and does not change her mind when Nancy’s classmate Elizabeth arrives, with Henry, from Arkham the next morning. The duo does not abandon Nancy, however. They investigate further and discover that Wilbur’s mother, Lavinia, is still alive and in an asylum. The town doctor, Dr. Cory informs Henry that Lavinia delivered twins when Wilbur was born, but one was stillborn, though he was not there for the delivery and never saw the body. The childbirth was very traumatic and Lavinia “lost her mind” during it, and nearly died.
In the meantime, on the advice of the locals, Elizabeth enters the Whateley house looking for Nancy. She opens a locked door, and releases a creature which appears to be Wilbur’s monstrous twin, who kills her and escapes. Upon Wilbur and Nancy’s return, Old Whateley confronts them about the presence of Nancy’s car, and in the ensuing argument, falls down the stairs and dies. Wilbur takes him to the local cemetery for a decidedly non-Christian burial, but the local townsfolk vociferously stop him.
Wilbur’s twin runs amok in Dunwich, killing several people. Lavinia dies in the asylum, looking much older than her 45 years. The Whateley estate burns down in a conflagration that may have to do with a pagan ritual. At the top of a coastal cliff, Wilbur prepares Nancy for sacrifice to bring back what he calls “The Old Ones.” Confronted by Armitage, Wilbur chants and calls down his demon father as his adversary chants reverse spells. Wilbur is struck by lightning in the ritual and falls in a ball of fire into the sea.
Finally, the physically unharmed Nancy is escorted off the sacrificial altar by Armitage and Cory, who calm her by stating that the Whateley line has ended. Nancy is pregnant, presumably with Wilbur’s ill-conceived child.