[4] Its plot follows a husband-and-wife team of doctors who attempt to open a rehabilitation center in a mansion built over a gateway to hell.

Arnold buys an abandoned mansion, once owned by a Civil War general named Emilio Vargas, which was built over hot sulfur pits.

's wife, Dr. Caroline Arnold, senses a presence that starts to manifest as a ghostly apparition.

attempts to pull him back to safety, but the rope becomes inexplicably hot before Pete's body spontaneously combusts and falls to the ground.

Meanwhile, Caroline witnesses the apparition of a man motioning for her to pick up an iron cross in the parlor.

She subsequently finds C.J., Raymond, and Felicia standing over Mary's corpse in a trancelike state; when she appears with the cross, however, it drives the three into a fit, and they return to consciousness, unaware of what just occurred.

A short time later, Laurie, Felicia, and Raymond attempt to melt the iron bars covering the windows with electrical cables.

Fulcrum of pic’s success or failure comes in final scenes, when Crenna and Pettet confront the devil himself, played with sinister angelicism by Victor Buono.

"[7] Linda Gross of the Los Angeles Times deemed the film "a scary and atmospheric thriller" with "excruciating suspense," also praising the performances and music.

[8] Bob Keaton of the Fort Lauderdale News noted the plot as "familiar," but conceded that "for those who enjoy the genre, The Evil is just their cup of horror.

"[9] Patrick Taggart of the Austin American-Statesman was less laudatory of the film, characterizing it as "yet another horror movie set in an old mansion...

The actors seem absolutely bored to death, as you likely will be after 20 minutes' worth of viewing...  Gus Trikonis directed with a flair that elevated tedium to the level of religion.