A loose adaptation of the short story of the same name by H. P. Lovecraft, the film concerns a young female graduate student who is targeted by a man attempting to use her in an occult ritual taken from the Necronomicon.

Though set in the fictional Massachusetts town of Arkham, principal photography of The Dunwich Horror took place in and around Mendocino, California in the spring of 1969.

At the Miskatonic University in Arkham, Massachusetts, Dr. Henry Armitage has just finished a lecture on local history and the very rare and priceless book known as the Necronomicon.

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.

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.

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.

"[5] Silver identifies one significant difference in the portrayal of Wilbur, who appears sensual and seductive in the film, whereas he is described in the story as "goatish" and uncharismatic.

[7] Originally, AIP had conceived an international co-production of the film between the United States and Italy, to be directed by Italian filmmaker Mario Bava and starring Boris Karloff and Christopher Lee.

[13] The role was Dee's first major part after the lapse of her years-long contract with Universal Pictures, and she saw it as a notable change of pace in her career.

[11][14] Dissatisfied with her previous work (Dee referred to her past 25 films with Universal as "all rotten"), she stated, "The reason I decided to do Dunwich was because I couldn't put the script down once I started reading it.

Principal photography of The Dunwich Horror took place in Mendocino County, California,[17] including the town of Little River, in the spring of 1969.

[21] John Duvoli of Cinefantastique noted that the film "is not Lovecraftian, but it is good technical film-making," also praising the special effects and score, but lambasting Dee's performance as ineffective.

[15] Ann Guarino of the New York Daily News awarded the film two-and-a-half stars out of four, noting that "Director Daniel Haller keeps interest high in the proceedings, but cannot avoid a letdown for, after all, he is dealing with the unbelievable.

"[22] Vincent Canby of The New York Times was less praiseful, writing that the film "has all the faults of Corman's various Poe adaptations (House of Usher, Pit and the Pendulum, The Masque of the Red Death), and very few of the virtues, with the exception of a beautifully Victorian interior set.

"[24] The Tampa Tribune's Sharon Cohen felt the film's special effects were unremarkable, and commented that all of the principal cast seemed out of place, "Ed Begley looks uncomfortable in his role... Sandra Dee, as the virgin, looks like a chubby coed whom someone found on a Hollywood movie lot.

And Dean Stockwell, with his curly long hair, looks vaguely like a warlock, but never manages to exude enough scary charm to be a convincing villain.

"[27] TV Guide awarded the film 2/4 stars, calling it "[a] fairly successful attempt at adapting H. P. Lovecraft for the screen.

[29] On his website Fantastic Movie Musings and Ramblings, Dave Sindelar stated that he disliked the changes to the film that departed from the original story, and criticized the performances, and underdeveloped characters.