American author Kenneth Magee is in England to meet his agent Sam Allyson. He sees a pretty blonde woman in the restaurant who seems to be watching him, but nothing comes of it. Over lunch Sam makes a $20,000 wager with Kenneth that he cannot write a gothic novel in twenty-four hours. Kenneth says he could if he had someplace quiet to work, where he would not be disturbed. Sam happens to know of an old manor house in Wales called Baldpate. No one has lived there for years and the property is soon to be sold. Kenneth accepts the bet and is soon on his way to Wales during a heavy rain.He cannot find the house and stops at a railway station to ask for directions. A young couple, Diana and Andrew Caulder, are stranded there but otherwise the place seems deserted. Suddenly a bent old woman wearing a hooded cloak enters. Upon seeing Kenneth, she goes into the ladies' restroom. Hearing the sound of broken glass, Diana goes to check on her but the room is empty. The old woman escaped by breaking a window. The stationmaster appears and points Kenneth in the right direction.The house is deserted just as Sam promised and has no electricity but Kenneth finds candles. He chooses a bedroom, sets up his typewriter, and goes to work. Suddenly the blonde woman from the restaurant appears and tells him he is in great danger. Kenneth thinks it is a trick set up by Sam to prevent him from working and orders her out. She leaves but returns and says someone is after her. She introduces herself as Mary Norton. This time Kenneth plays along and they search the upstairs. Along the wall are gloomy portraits of the Grisbane family, apparently the last owners. Where Roderick Grisbane's portrait should be is an empty space.They smell food cooking and go downstairs. An elderly couple are there who claim to be the caretakers. But Sam had said there were no caretakers. Kenneth doesn't understand why the electricity is now on but the woman says it was on all the time. She admits they are not the caretakers but rather Lord Grisbane and his daughter Victoria. They have been away from the house for 40 years but have now returned to meet the other Grisbanes; Victoria's brothers Lionel and Sebastian.Mary and Kenneth return to his room and she takes a nap. Kenneth works for a while but can't shake the feeling that something sinister is afoot. Then Diana and Andrew Caulder appear, admitting that they followed Kenneth because they didn't want to spend the night in the train station. Kenneth once more tries to work but Lionel and Sebastian arrive. Everyone sits down to dinner and Lionel explains they are here to honor a pact they made 40 years ago. He hints at a scandal involving Roderick but is not specific.After dinner Victoria entertains the group with a gloomy song, accompanying herself on the piano. She breaks off to cry out that she was once beautiful and engaged to be married. But her family refused to permit it. Her father and brothers look very uncomfortable.A man knocks at the door and introduces himself as Corrigan. He demands to know what they are all doing there and threatens to call the police. Kenneth explains his presence but the Grisbanes evade the question. At last they admit that their youngest brother Roderick got a village girl pregnant when he was a teenager. He then murdered her. Instead of turning him in to the police, the Grisbanes decided to imprison him in his bedroom for 40 years. They made a pact to return on this night to unlock the door. Corrigan is outraged and says they are monsters. Everyone goes to Roderick's bedroom but it is empty.At this point Kenneth, Mary, and the Caulders decide to leave. But their tires have been slashed, as have Corrigan's. Diana is furious and blames the whole thing on Andrew. They argue bitterly and she runs sobbing upstairs. When she goes to wash her face, the basin is filled with acid instead of water. She dies a horrible death.Believing that Roderick, now a crazed madman, is loose in the house, everyone separates and tries to find him. One by one, the Grisbanes (all but Lionel) and Andrew are murdered. There remains only Lionel, Corrigan, Kenneth and Mary. Corrigan announces that he is actually Roderick Grisbane. He escaped and made a life for himself but returned tonight to kill his family. He says it was really Lionel who impregnated and murdered the village girl but his family decided to let him take the blame. Lionel was the oldest and they wanted to prevent a scandal. Corrigan kills Lionel and then advances on Kenneth and Mary. Kenneth fights and kills him. Then Mary smiles, kisses Kenneth on the cheek and walks away. Suddenly all the characters come back to life and gather in the main hall. A spooky laugh is heard and Sam enters.He set the entire thing up and hired actors. Mary is actually married to the actor playing Andrew Caulder. Kenneth is dazed but admits that Sam put one over on him. Food and drinks are served and everyone mingles.The scene switches to Kenneth's bedroom, where he is typing the last sentence in his novel. It is daylight and he has won the bet. The Grisbanes, Mary, and the Caulders exist only in his imagination. What happened the night before was actually the plot of his novel. He drives away and the house is left undisturbed.Sam meets him for lunch and Kenneth hands him the novel. But he tears up Sam's check. He notices that one of the waiters strangely resembles Lionel Grisbane. On his way out of the restaurant, he runs into the blonde woman. He put her into the novel as Mary Norton but she is actually Sam's secretary.