In the opening scene, Sam (Ed Bakey) a caretaker to a large abandoned mansion, enters to clean up for a potential visit from a realtor when something scares him so that he catches on fire and flees outside where he burns to death.Some time later, psychiatrist C.J. Arnold (Richard Crenna) buys the abandoned mansion, once owned by a Civil War general named Emilio Vargas, which was built over hot sulfur pits. There, he plans to set up a drug rehab center. C.J. recruits a group of volunteers to help clean up and renovate the large house. The volunteers include C.J.'s wife Dr. Caroline Arnold (Joanne Pettet), Professor Raymond Guy (Andrew Prine), Mary Harper (Cassie Yates), Peter Brooks (George O'Hanlon Jr.), Felicia Allen (Lynne Moody) and Laurie Belman (Mary Louise Weller) Almost immediately, Caroline senses a presence that starts to manifest as a ghostly apparition. Soon thereafter, more strange and eerie things start to happen which start to agitate the volunteers, along with the resident dog.Later on, C.J. discovers a trap door in the basement, which he opens up and unknowingly unleashes a menacing evil spirit. Suddenly, all of the doors and windows become locked, trapping everyone inside the mansion. They soon discover that the trap door in the basement is actually a gateway into hell. While Felicia is sleeping, she is suddenly attacked by an invisible force that beats her and tears her clothing off. Her screams are heard by the others, who come to console her. The group attempt to find a way out of the house, and decide to scale the wall from a turret on the house. Peter attempts to descend using a rope, but is overtaken by an invisible force. C.J. attempts to pull him back to safety, but the rope becomes inexplicably hot, before Pete's body spontaneously combusts, and falls to the ground, dead.Further attempts to escape are fruitless, including the men attempting to saw the door open with a skill-saw, as the blade inexplicably wears down rapidly, inflicting no damage on the wood. While doing so, Raymond inadvertently saws through his own hand. Mary flees upstairs to retrieve bandages. She is attacked by the dog in the hallway, who lunges at her, knocking both over the staircase railing, causing them both to fall to their deaths.C.J. begins storing all of the deceased's bodies in an enclosed porch, but shortly after finds that Mary's body has vanished. Meanwhile, Caroline witnesses the apparition of a man motioning for her to pick up an iron cross in the parlor. When she does, the apparition vanishes. She subsequently finds C.J., Raymond, and Felicia standing over Mary's corpse in a trance-like 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. During the attempt, Laurie is dragged away upstairs by a dark unseen entity (Emory Souza). Raymond chases after her, but is thrown down the staircase by the demon. In a panic, Raymond and Felicia resume their endeavor, but Felicia is killed when Mary's corpse reanimates, startling her and causing her to fall back against the iron bars, electrocuting her to death.The electricity causes the bars in one of the windows to melt, and Raymond leaps through the window. As he runs away from the house, however, the ground beneath him turns to quicksand, and he sinks into the mud, drowning.Meanwhile, Caroline becomes possessed by the ghost of Emilio Vargas (Galen Thompson), whose apparition she has repeatedly seen; Vargas, using Caroline's body as a conduit, tells C.J. he has released "the evil" in the house by unlocking the pit. Together, they descend into the basement to close the pit, but are overcome by a powerful force that knocks Caroline into the pit. C.J. descends into the pit to get her, and the two find a series of tunnels.C.J. enters a white cavern, where he is confronted by the Devil (Victor Buono), appearing as a bearded, chubby, middle-aged man wearing a white suit, who threatens and belittles him. Caroline appears with the cross, stabbing it into the Devil's chest, and together she and C.J. flee, escaping the pit and sealing it closed. When they do, the windows and doors of the house miraculously unlock. They flee outside into the morning light of dawn, while Vargas' spirit watches from a window.