Eccentric millionaire Frederick Loren (Vincent Price) invites five people to a party he is throwing for his fourth wife Annabelle (Carol Ohmart) in an allegedly haunted house he has rented, promising to give each $10,000 with the stipulation that they stay the entire night in the house after the doors are locked at midnight. The guests are test pilot Lance Schroeder (Richard Long), newspaper columnist Ruth Bridges (Julie Mitchum), psychiatrist Dr. David Trent (Alan Marshal) who specializes in hysteria, Nora Manning (Carolyn Craig) who works for one of Loren’s companies, and the house’s owner Watson Pritchard (Elisha Cook). All are strangers to both the Lorens and each other, with their only commonality a desperate need for money.

The Lorens have a tense relationship; Frederick is convinced Annabelle tried to poison him in order to acquire his wealth, which Annabelle firmly denies, attributing his suspicions to paranoia and jealousy. Pritchard believes that the house is genuinely haunted by the ghosts of those murdered there, including his own brother. He gives a tour of the house, including a vat of acid in the basement which was used by a previous resident to kill his wife. When Schroeder and Manning remain behind to further explore the basement, Schroeder is locked in an empty room and struck on the head, while Manning is confronted by a menacing ghost.

Annabelle privately warns the guests that her husband is scheming something, and that she suspects him of murdering his second and third wives. Gathering downstairs, the guests are told the rules of the party, and each is given a .45 ACP caliber pistol for protection. Having encountered further apparitions, Manning decides against staying the night, but the caretakers lock the doors five minutes early, taking that option out of the guests’ hands.

Schroeder is confronted by Manning, who tells him an unseen assailant strangled her and left her for dead. In light of Annabelle’s warnings they both suspect Loren. He tells her to remain out of sight so that her attacker will still think her dead. Hearing a scream, Schroeder and Trent find Annabelle’s corpse, suspended to suggest she hanged herself, but the absence of a perch immediately arouses suspicions of murder. To survive the night, Schroeder and Trent propose that everyone stay in their rooms and shoot anyone who enters; thus the innocents will have no reason to leave their rooms, and the killer must stay put or admit his guilt.

Manning is chased from her room into the basement by Annabelle’s ghost. Aroused by the ghostly sounds, Trent concludes that the killer is about and proposes he and Loren split up to search the house. Trent instead meets with Annabelle, who faked her death using a hanging harness and sedatives. Secretly lovers, the two of them orchestrated the various mishaps in order to manipulate Manning into killing Loren. Manning, seeing Loren enter the basement with a gun in his hand, does indeed shoot him. After she flees, Trent slips in to dispose of Loren’s body in the vat of acid.

Annabelle walks to the basement to confirm her husband is dead. A skeleton rises from the acid, accuses her in Loren’s voice, and shoves her into the vat. Loren emerges from the shadows, holding the contraption that he used to manipulate the skeleton, now revealed to belong to Trent.

Manning tells the other guests that she shot Loren. When they arrive, Loren explains that he had their guns loaded with blanks, that Trent and his wife plotted to kill him, and that they both met their end in the vat of acid, though he lies to conceal the fact that he murdered his wife, claiming she stumbled and fell into the vat. Pritchard remains convinced the house is haunted, with Trent and Mrs. Loren now added to the ranks of its ghosts.