The film is set in an abandoned asylum, the Vannacutt Psychiatric Institute for the Criminally Insane, where numerous murders were committed in the 1940s and 50s. The head of the facility, Dr. Richard B. Vannacutt, performed grotesque experiments and medical procedures on the patients, killing many in the process. The hospital was closed when some of the so-called "patients" escaped, killing almost the entire staff and burning the hospital. Vannacutt had rigged the building with numerous iron gates, activated by cranks and levers, to serve as barriers to keep patients from leaving the building, should they escape; some of these were controlled by huge, clock-like timers that wouldn't open for twelve hours. During the fire, he released these gates, keeping the inmates, employees and the fire itself contained. After several unexplained deaths during reconstruction of the facility, it was dubbed "The House on Haunted Hill".
Present day. Stephen Price is an eccentric amusement park mogul whom has a disintegrating marriage to Evelyn, his spoiled trophy wife. Stephen's wicked sense of humor has caused a strain between him and his wife, each of whom would gladly kill the other. Evelyn fancies spectacular parties, so Stephen leases the house from the owner, Watson Pritchett, for Evelyn's Halloween birthday bash. Evelyn gives Stephen a guest list two pages long; he shreds it to spite her and then creates one of his own. The five people who show up for the party: model Jennifer Jenzen (aka: Sara Wolfe), Eddie Baker, Melissa Margaret Marr, Dr. Donald Blackburn and Pritchett himself, aren't the ones that he invited. Neither Evelyn nor Stephen know who they are. Despite this, Price continues the party's theme, offering a million dollars to anyone who stays in the house and survives until morning, with any person not making it having his money added to the pot.
Shortly after arriving, the security gates are tripped, locking everyone inside and forcing them to remain there until the gates unlock in the morning. Price scolds Carl Schecter--a company employee, who has developed a series of harmless traps to scare the guestsfor not letting him know he planned to pull a stunt like that. Schecter said it wasn't him who did it. Price is then convinced his wife did it somehow. Shortly after, Melissa is killed by one of the ghosts when she wanders off into the basement of the house. It's revealed that the spirits themselves created the guest list specifically to include the descendants of five members of Vannacutt's staff who didn't die in the long-ago fire. Also, it is revealed that Evelyn and Blackburn are in working together to bring down Stephen. Evelyn then kills Blackburn and uses his body to frame Stephen.
After a deranged Stephen attempts to kill Evelyn for orchestrating his murder plot, the two accidentally unleash the Darkness of the house when he throws her through an ancient and decayed door. The Darkness is a dark, shape-shifting creature composed of all the spirits in the house, led by Dr. Vannacutt; this force comes after and tries to kill all the remaining guests. It starts by assimilating Evelyn into itself, killing her while Price watches in horror.
The Darkness reveals how the evil souls that comprise it want to feed on all those "who are responsible". Upstairs, Pritchett, Eddie and Sara are trying to open one of the iron gates on the windows when they hear Price's screams from down the hall. Pritchett investigates, while the Darkness catches up with Price on the other side of the door; he jumps out of the way at the exact same moment when Pritchett opens the door and the Darkness kills him and flows away. Price runs through the hall while Sara and Eddie follow, trying to figure out how he's alive when Sara supposedly shot him (answer: a bullet-proof vest). Price tells them that what Pritchett's been saying all along is true: the house is alive and killed everyone. He figures that the only way to get out is to go to the attic and try and escape through there. Price runs ahead of them to activate them while Sara and Eddie stay behind. The Darkness seeps through the house and tries to kill them.
As they are running up the stairs to the attic Sara trips, and the Darkness uses Melissa's form to try to lure her to it. Price by then has activated a pulley that reveals an opening in the window of the attic. Eddie and Sara get there, while the Darkness seeps up the cracks of the wooden floor, and Sara lingers while Eddie goes to the opening. Eddie looks back from the window to find that the Darkness had arrived and is attempting to sever the rope that keeps the iron gate open. While Eddie rushes back to get Sara, the Darkness tries to kill her but instead kills Price, who had sacrificed his life to get Sara out of the way. Sara escapes just as the iron gate comes crashing down, trapping Eddie with the Darkness.
The Darkness then confronts Eddie and brings up charges against his ancestor's actions. Eddie then screams that he was actually adopted. At the sound of this, Pritchett's ghost appears and frees Eddie by pulling the rope needed to open the iron gate, at which Sara pulls Eddie through as the Darkness is about to kill him. After he is pulled through, the gate shuts, Pritchett's ghost disappears and the Darkness disappears into nothingness. As Sara and Eddie sit in exhaustion over the night's events, they notice that an envelope has been pushed through the gate, containing five checks for $1 million each. They smile and enjoy the rays of the sun. It ends with Sara asking, "One more thing, how do we get down from here?" as they are several hundred feet above the ground and there's no exit.