The leading role was offered to Peter Fonda, but he turned it down. Instead, Dean Stockwell played the role of Wilbur Whateley. The film was shot in Mendocino, California.
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 (Begley) 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 (Dee) to return to the library. She is followed by a stranger, who later introduces himself as Wilbur Whateley (Stockwell). Whateley 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 Whateley's hypnotic gaze.
Whateley's perusal of the book is cut short by Armitage, who has researched Wilbur's family's sordid past. His warnings about the Whateleys 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 (Jaffe), Wilbur's grandfather. Her car is then disabled and she is drugged by the younger Whateley. She decides under the influence of hypnosis and drugs to spend the weekend, and does not change her mind when Armitage and Nancy's classmate Elizabeth arrive from Arkham the next morning. The duo does not abandon Nancy, however. They investigate further and discover that Wilbur's mother, Lavinia (Joanna Moore Jordan) is still alive and in an asylum. The town doctor, Cory (Bochner) informs Armitage 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 the missing girl'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.
American International Pictures originally announced filming the novel in 1963.
Sandra Dee was paid $65,000 plus 5% of the profits. This was the last film of actor Ed Begley, who died 3 months after its theatrical release.
The film was released on DVD by MGM on August 28, 2001. It was re-released again by the company as a part of a multi-disk set on September 11, 2001 and as a double feature with Die Monster, Die! on September 20, 2005.
Leonard Maltin awarded the film 2 1/2 stars out of 4 stating that the film was "often effective", but also stated that the film's ending "ruined the whole movie".
AllMovie gave the film a mixed review stating " Everything about the film -- the performers, the hair styles, the psychedelic imagery, the music -- has late-'60s tackiness written all over it, which leaves it very dated and not very Lovecraftian". TV Guide awarded the film 2 out of a possible 4 stars, calling it "[a] fairly successful attempt at adapting H.P. Lovecraft for the screen." It currently has a rating of 17% "Rotten" on film review aggregate Rotten Tomatoes with an average rating of 4.5/10 based on 6 reviews.
Another film version, produced by Active Entertainment Finance and Bullet Films, was released in 2009.