Frederick Loren (Vincent Price) is an eccentric millionaire who has rented a Gothic stone house on a hill overlooking Los Angeles for a "haunted house" party that his fourth wife Annabelle (Carol Ohmart) had decided to throw. Over the opening credits, five people arrive at the house being driving in "funeral cars". They are: test pilot Lance Schroeder (Richard Long), magazine columnist Ruth Bridges (Julie Mitchum), psychiatrist Dr. David Trent (Alan Marshal), the house owner Watson Pritchard (Elisha Cook Jr.), and secretary/typist Nora Manning (Carolyn Craig). Except for the owner and for Dr. Trent (who wants to investigate the supernatural), they were all chosen because they wanted/needed the $10,000 that Loren was offering to anyone who could stay one night in the house. Add these five to Frederick and Annabelle Loren, and there are seven people in all -- three women and four men. Of course, some of them had underlying motives, too.
During the gathering in the parlor, Watson Pritchard claims that seven people, three women and four men, including Watson Pritchard's brother, have been murdered in the house. Parts of the bodies were found all over the house, but the heads have never been found. Pritchard himself spent a night in the house and was found almost dead the next morning. He claims that the heads can be heard at night whispering to each other.
Fredrick Loren enters who introduces himself and claims that he invited them to spend one night the house, with the stipulation that the power will be out and all doors will be locked at midnight, allowing no accessible escape. Anyone who stays in the house for the entire night, given that they are still alive, will each receive $10,000.
He explains the rules of the party and gives each of the guests a .45 caliber pistol for protection. Loren's wife tries to warn the guests that her husband is psychotic, causing them to be very suspicious of him, especially Nora Manning, who becomes convinced that he's trying to kill her when she keeps seeing mysterious ghouls, including the ghost of Annabelle, who had hanged herself after being forced to attend the party.
After being driven into a fit of hysteria by the ghosts haunting her, Nora shoots Mr. Loren, assuming he is going to kill her. Dr. Trent, another guest, tries to get rid of the body by pushing it into acid, but the lights go out, and when they come back on, both of the men are gone.
Annabelle emerges, having faked her death with the help of Dr. Trent, and having apparently tricked Nora into killing Loren. Suddenly, a skeleton emerges from the acid accompanied by the voice of Loren. The specter approaches Annabelle as she recoils in terror. In this panic, the screaming Annabelle accidentally backs into the acid herself. The real Mr. Loren walks out of the shadow, holding the contraption that he was using to control the skeleton of Dr. Trent. In his triumph, he watches Annabelle disintegrate.
"Good night, Doctor. Good night, Annabelle," Frederick Loren says to himself. "The crime you planned was indeed perfect, only the victim is alive and the murderers are not. It's a pity you didn't know when you started your game of murder that I was playing, too." Loren turns himself over to the other guests, informing them that Nora's gun was loaded with blanks and that Trent and Annabelle were conspiring to kill him.
In the final scene, Prichard looks into the acid vat and says, with a terrified expression on his face, "Now there are nine. There will be more...many more. They're coming for me now. And then they'll come for you."