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One rainy night, Lois Garland, a young textile designer, knocks on the door of psychiatrist Robert Ordway, desperately seeking the doctor's help. After relating her terrifying nightly dream of a ghostly woman who beckons Lois to follow her to the rocky shore, Lois pleads with Ordway to come to her isolated home at Rocky Point to investigate. When Lois rejects the doctor's advice to check into a hotel and returns home instead, he drives out to Rocky Point the next evening. There he meets the inhabitants of the house that Lois operates as a hotel for her relatives and friends: Nick Kallus, the cook; Lois' sister, Adele Carter, and her husband Stanley, a failed actor; her uncle Frank Swift; Jess Hilton, the junior partner of Lois' employer Raymond Shields; Lois' lawyer Frederick Gordon and the caretakers of the property, Mr. and Mrs. Riggs. As they prepare for bed that night, Ordway suggests switching rooms with Lois and after the doctor falls asleep, a cloud of gas seeps through the ventilator above the bed, putting him into a hypnotic trance. A ghostly figure then enters the room and leads the doctor out of the house and onto the rocky cliffs above the sea. When Ordway collapses, Swift, spying his prone figure from a distance, helps the doctor back to the house. After they part in the main hallway, Ordway stumbles upon a man's body lying on the stairs. Ordway then hurries to Lois' room, hoping that she can identify the victim, but by the time they return to the stairway, the corpse is gone. While strolling on the beach the next morning, Ordway finds the man's body in the sea and Lois identifies him as Shields, who was supposed to be in town on business. The sheriff is then summoned, but is skeptical of Ordway's claims that it was Shields's body he saw in the hallway the previous evening. That night, Ordway ventures into Swift's laboratory and Swift, an industrial chemist, explains that he is developing a new formula for a synthetic fabric. When Ordway drops the test tube containing the compound, it breaks, releasing a gaseous cloud. Meanwhile, in the living room, Lois and Jess, who are in love with each other, discuss the fact that Shields's death removes an obstacle to their romance because Shields had proposed to Lois. After leaving the laboratory, Ordway goes to find Gordon, and discovers a photo of Carter and his son. Gordon then explains that Adele is Carter's second wife and that the boy is the offspring of his first marriage to the daughter of a steel magnate. From Gordon's room, Ordway descends into the cellar and finds a laundry chute and a set of stairs leading to a cave that opens to the sea. After pocketing a candlestick he found in the passageway, Ordway climbs back upstairs and is greeted by a gun-wielding Nick. After Ordway disarms him, Nick warns the doctor that Lois will be accused of Shields's murder unless Ordway testifies at the inquest that the death was an accident. Proceeding to Lois' room, Ordway shows her the candlestick and relates his confrontation with Nick. Lois responds that although Nick served a year in prison for killing a man, he is harmless. She then asks Ordway not to mention the candlestick for fear that Jess will be accused of Shields's murder. That night, as Ordway sleeps, a figure wearing a gas mask sneaks into his room and removes the candlestick. Riggs, passing by, hears the disturbance, and when he enters the room, the figure strikes and kills him with the candlestick. Upon awakening, Ordway discovers Riggs's body and notifies the household and the sheriff that another murder has been committed. When the sheriff arrives, Ordway postulates that Riggs was killed because he was about to divulge an important clue and Shields was killed when he interrupted the person controlling the hypnotic gas. While the sheriff searches the cellar, Ordway climbs into the attic to see if the murderer could have hidden there and overhead his conversation with Lois about the candlestick. There he finds an envelope addressed to Gordon that contains a white powder that turns into a gas when exposed to the air. When Ordway questions Gordon about the envelope, Gordon states that he discarded the envelope after extracting a letter concerning Carter's custody battle for his son. Gordon's news agitates Lois, who claims that she had threatened to expose Carter as an unfit father for stealing charity funds if he ever tried to win custody of his son. As the sheriff prowls the cellar, a wig, mask and candlestick are dropped down the laundry chute. Hurrying upstairs with the evidence, the sheriff shows it to Ordway and then finds Carter bound and locked in a closet just outside the bathroom containing the laundry chute. After Carter claims that someone knocked him unconscious and then locked him in the closet, the group reassembles in the living room. There Ordway asks Nick to demonstrate how someone can easily tie themselves up. After exhibiting the key that he found in the closet, Ordway produces a pair of Carter's pants, which have pitch stains on the knees, and explains that the pants were stained as Carter bent over the attic ventilator above Lois' room. When Ordway concludes that Carter was trying to drive Lois insane so that she would be unable to testify against his custody suit, Carter tries to flee but is shot down by the sheriff.