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Wealthy former playboy Frederick Loren rents the haunted mansion of Watson Pritchard for one night and invites five strangers on the pretense that he is throwing his wife Annabelle a party.  Frederick bribes the guests, test pilot Lance Schroeder, newspaper columnist Ruth Bridges, physician David Trent, secretary Nora Manning and Watson, by promising ten thousand dollars to each if he or she spends the entire night at the mansion.  Although puzzled by the invitations, the guests are in need of the money and agree to participate.

Shortly after their arrival, Lance prevents Nora from being struck by a falling chandelier.  Watson then reveals that there have been seven inexplicable, brutal murders in the house, including that of his brother, and he believes the victims’ ghosts continue to haunt the premises.  In private, Frederick and Annabelle argue about the party, which Frederick admits is a ploy to force Annabelle into revealing that she only wants access to his fortune.

Later, Frederick introduces himself to the others and explains that at midnight the servants will lock them into the mansion for twelve hours and that all communication with the outside will be cut off.  Watson gives the guests a tour of the house, detailing the gruesome deaths that occurred in several rooms, including the wine cellar where a jealous husband murdered his wife by pushing her into a vat of acid that still exists beneath a large trap door.

When the others leave the cellar, Lance and Nora linger behind to explore.  After Lance abruptly disappears into a small room, Nora is frightened by the vision of a wild-haired old woman.  Lance is discovered knocked out but unharmed in a small room and David treats a cut on his head.  Determined to learn what happened in the cellar, Lance returns to the basement later with Nora, where she sees the woman again, who disappears before Lance can see her.  Frustrated that Lance does not believe her, Nora returns upstairs where she meets Annabelle, who cautions Nora and Lance that she believes Frederick may be trying to harm her.

Moments later, Nora returns to her room and is horrified to find a severed head in her suitcase.  When she runs into the hall panicked, a dark figure grabs her and cautions her to leave before she is murdered.  Nora hastens to the living room where Frederick introduces the others to the servants, Jonas Styles and his wife, who Nora recognizes as the man in the hallway and the woman in the cellar.  Nora refuses to remain in the house and, as it is a few moments before midnight, Frederick agrees that she may depart.  Abruptly, however, Jonas and his wife bolt, locking the group into the house.

Annabelle enters the room and apologizes to her guests, then asks Frederick to pay them and allow them to depart.  Instead, Frederick gives each guest a party favor of a miniature casket with a pistol inside.  Nora demands that Watson and the others see the severed head, but when they arrive in her room it is gone.  David offers Nora a sedative, but she angrily refuses.

Later, Lance visits Nora only to find her room empty, but he discovers the severed head in her closet.  Lance hurries to show the object to Watson, but the men are interrupted by a scream and discover a woman’s body hanging in the hallway.  Lance believes it is Nora, but upon lowering the body, the men realize it is Annabelle.

Afterward, Lance discovers Nora hiding in his room and gives her his gun, advising her to remain hidden there.  The others meet in the living room and Frederick asks David if Annabelle committed suicide or was murdered.  David insists that they contact the police immediately, but is reminded the phone lines have been cut.  Lance suggests one of the group is a murderer and advises that they return to their rooms and remain there until morning.  Lance does so and tells Nora that he believes Frederick murdered Annabelle and that he intends to break out and bring the police.

After Lance departs, a thunderstorm begins and Nora is terrified when a large rope slithers into the room through the window and she sees Annabelle’s figure outside at the end of the rope.  Nora bolts and runs down the hall where she discovers Annabelle’s body hanging from the rafters.  Thoroughly frightened, Nora flees downstairs as David, suspicious of the noise, visits Frederick to suggest they search the house.  After Frederick departs, David goes to his room and rouses Annabelle, who has been shamming death in a plot with David to drive the edgy Nora to murder Frederick.

Meanwhile, searching for Lance, Nora returns to the cellar and, when Frederick appears moments later, the terrified Nora shoots him and flees.  Hearing the disturbance, David appears and prepares to dump Frederick into the acid vat, when Frederick revives and pushes David into the acid.  Moments later, Annabelle arrives searching for David and is horrified when a skeleton rises out of the acid, approaches her, and in Frederick’s voice accuses her of murder.  After the skeleton pushes the terrified Annabelle into the vat, Frederick steps out of the darkness revealing a complex set of wiring with which he controlled the skeleton.  Meanwhile, Nora seeks out Ruth and Watson and they discover Lance trapped in a hidden passageway.  Nora confesses that she has shot Frederick but when the group returns to the cellar, the wounded Frederick admits his actions and surrenders, prepared for the law to judge him.