Physicist Lionel Barrett is enlisted by an eccentric millionaire, Mr. Deutsch, to make an investigation into "survival after death" in "the one place where it has yet to be refuted." This is the Belasco House, the "Mount Everest of haunted houses," originally owned by the notorious "Roaring Giant" Emeric Belasco, a six-foot-five perverted millionaire and supposed murderer, who disappeared soon after a massacre at his home. The house is believed to be haunted by numerous spirits, the victims of Belasco's twisted and sadistic desires.
Accompanying Barrett are his wife, Edith, as well as two mediums: a mental medium and Spiritualist minister, Florence Tanner, and a physical medium, Ben Fischer, who is also the sole survivor of an earlier investigation. The rationalist Barrett is rudely sceptical of Tanner's Christian faith and spiritual beliefs, asserting that there is nothing but unfocused electromagnetic energy in the house. Barrett brings a machine he has developed, which he believes will rid the house of any paranormal presence or force.
Though not a physical medium, Tanner begins to manifest physical phenomena inside the house. When, after a quarrel with Tanner, Barrett is attacked by invisible forces, he suspects that Tanner may be using the house's energy against him. Meanwhile Fischer remains aloof, with his mind closed to the house's influence, and is only there to collect the generous pay offered him to return.
Edith Barrett is subjected to erotic visions late at night, which seem linked to her lackluster sex life. She goes downstairs and, in a seeming trance, disrobes and demands sex from Fischer. He instead strikes her, snapping her out of the trance, and she returns to herself, horrified and ashamed. A second incident occurs a day or so later (this time, she is awake but uninhibited due to alcohol); her husband arrives a moment later to witness her advances to Fischer and is resentful, stating to Fischer's face that he believes that Fischer no longer has any psychic ability and that "Mr. Deutsch is wasting one-third of his money!" Stricken by the accusation, Fischer finally drops his psychic shields but is immediately attacked.
Tanner, convinced that one of the "surviving personalities" is Belasco's tormented son Daniel and determined to prove it at all costs, finds a human skeleton chained behind a wall. Believing it to be Daniel, Tanner and Fischer bury the body outside the house and Tanner performs a funeral. Despite this, Daniel's personality continues to haunt Tanner; she is scratched violently by a possessed cat and Barrett, seeing the scratches, suspects that Tanner may be mutilating herself. In an attempt to put the supposed Daniel to rest, Tanner gives herself to the entity sexually, and later appears to be possessed herself, temporarily.
Barrett's machine is assembled. Tanner attempts to destroy it, thinking that it will harm the spirits in the house, but is prevented. She enters the chapel, the unholy heart of the house, in an attempt to warn the spirits, and is crushed by a falling crucifix. (During her dying moments, she leaves a clue written in her own blood, to the true source of the haunting, which she now knows.) Barrett meanwhile activates his machine, which seems to be effective. Finally activating his psychic abilities as he wanders in the house, Fischer declares the place "completely clear!" in astonishment. However, soon afterwards, violent psychic activity resumes and Barrett is killed.
Fischer decides finally to confront the house, with Edith accompanying him despite her misgivings. In the chapel, a confrontation ensues: thanks to clues from the manner in which Tanner, Barrett and the previous investigators had died, Fischer deduces that Belasco is the sole entity haunting the house, masquerading as many. He taunts Belasco, declaring him a "son of a whore", and that he was no "roaring giant", but likely a "funny little dried-up bastard" who fooled everyone about his alleged height. Even as objects begin to hurl themselves at Fischer, he continues to defy the entity, until all becomes still, and a portion of the chapel wall shatters, revealing a hidden door.
Going inside, Fischer and Edith discover a lead-lined room, containing Belasco's preserved body seated in a chair. Pulling out a pocketknife, Fischer rips open Belasco's trouser leg, discovering his final secret: a pair of prosthetic legs. Fischer and Edith realise Belasco had had his own stunted legs amputated, and used the prosthetics in a grotesque attempt to appear imposing. Belasco had the lead lined room specially built (Belasco himself possibly suspecting the electromagnetic nature of life after death that Barrett's theories predicted), in the event of his death, to preserve his spirit, afraid of what may happen otherwise.
With the room now open, Fischer activates Barrett's machine a second time, and he and Edith leave the house, expressing hopes that Barrett and Tanner will guide Belasco to the afterlife without fear.