Out of work following an appendectomy, Julia Ross desparately scans the classified ads in search of a job. Attracted by a notice from the Allison Employment Agency, Julia proceeds to their office, where she is interviewed by Mrs. Sparkes, who is looking for a private secretary for Mrs. Williamson Hughes. Upon ascertaining that Julia has no family, Mrs. Sparkes hires her and instructs her to move into the Hughes home that evening. After Julia leaves the office to pack her bags, Mrs. Sparkes informs Mrs. Hughes that now that they have found a candidate, they can close the agency. As Julia gathers her belongings at Mrs. Mackie's rooming house, she runs into her former beau, Dennis Bruce, and they make a date for the following evening. Before leaving, Julia hands Bertha, Mrs. Mackie's maid, her back rent and a forwarding address. Bertha, who thinks that Julia is condescending toward her, pockets the money, however, and tears up the address. That night, while Julia slumbers at her new residence, her purse and clothing are removed from her room and destroyed. After Julia fails to keep her date with Dennis, he goes to the Hughes's house and is informed by a police officer that Mrs. Hughes and her son Ralph have moved. When Dennis questions Mrs. Mackie about Julia's forwarding address, Bertha admits to tearing up the paper that Julia entrusted to her. Bertha mentions that Julia got her job through the Allison Employment Agency, so Dennis visits the agency and discovers that it has closed. The next day, Julia awakens in a strange room and discovers that she is dressed in a nightgown monogrammed with the initials "M. H." Soon after, a nurse named Alice enters her room and addresses her as Mrs. Hughes. When Julia becomes agitated, the nurse summons Julia's "husband", and Ralph and his mother appear at the doorway and call her Marion. After Julia demands an explanation, Mrs. Hughes states that Julia is mentally deranged and believes that she is someone else. When Mrs. Sparkes enters the room and identifies herself as the Hughes's housekeeper, Julia realizes that she is being held prisoner in a sinister seaside mansion. That night, Julia is awakened by a shadowy hand, and when she sees a man's reflection in her bedstand mirror, she screams. Drawn by Julia's shrieks, Mrs. Hughes appears at the door and insists that Julia was simply startled by a prowling cat. After leaving Julia's room, Mrs. Hughes warns Ralph to be more prudent and confiscates a pocket knife that he has been nervously fingering. The next morning, Julia bolts out the front door and runs toward the gate, where she is stopped by the gatekeeper. When the man leaves his post to report to the house, Julia pencils a plea for help on a scrap of newspaper, wraps it around a rock and tosses it onto the road. Later, a car pulls into the Hughes's driveway, and Julia, thinking that it is the police answering her summons, eagerly greets them. Her hopes are dashed when the occupants of the car introduce themselves as Reverend Jonathan Lewis, his sister, Mrs. Susan Robinson, and his sister's husband. When Julia blurts out that she is being held prisoner, Mrs. Hughes declares that she is hysterical and suffering from a nervous breakdown. On the drive back to town, the reverend realizes that Julia is hiding in his backseat, turns the vehicle around and delivers her to Ralph. Back at the house, Mrs. Hughes and Ralph are plotting how to make Julia's murder appear as suicide when Mrs. Hughes discovers a plea for help that Julia has addressed to Dennis. After Mrs. Hughes removes the letter from its envelope and replaces it with a blank piece of paper, Ralph decides to mollify Julia by agreeing to drive her into town so that she can post the letter. Julia outsmarts them however, and discards the blank sheet, replacing it with another entreaty for help. Upon returning home that night, Julia hears the cat crying and finds him trapped behind a panel in the wall leading to a secret passageway. After taking a few steps into the passageway, Julia overhears Mrs. Hughes planning to murder her that evening. Ralph, who married his wife Marion for her money and then slashed her to death with his knife, is reassured by his mother that Julia's murder will be made to look like Marion's suicide. Desperate, Julia staggers back to her room and finds a bottle of poison. Upon discovering Julia's unconscious body, Mrs. Hughes calls a doctor to treat her, and when Julia regains consciousness, she tells the doctor that she did not ingest the poison. After informing the physician that Ralph killed the real Marion, she recounts sending the letter and asks him to hide her until Dennis arrives. The doctor is an impostor, however, and is really a man named Peters in the employ of Mrs. Hughes. After Mrs. Hughes sends Peters to London to intercept the letter, the real doctor arrives and Mrs. Hughes explains that Julia is insane. When the physician advises institutionalizing her, Mrs. Hughes asks him to return the next day. Meanwhile, Peters arrives in London and gains entry into Mrs. Mackie's boardinghouse by feigning interest in renting a room. After pocketing Julia's letter, he leaves, but as soon as Mrs. Mackie realizes that a letter is missing, she calls the police. That night, as Julia awaits her demise, she blockades the door to her room and then pries the bars from her window. When she screams, Mrs. Hughes and Ralph break down the door and see her figure splayed on the rocks below. At the break of dawn, the doctor arrives with his nurse and Mrs. Hughes informs him that Marion has committed suicide. To make sure that Julia is dead, Ralph runs to her body and is about to bludgeon her with a rock when a police officer, accompanied by Dennis, appears and announces that he is under arrest. After Julia climbs to her feet and explains that the figure he saw was only her robe and that she hid in the secret passageway until Dennis and the police appeared, Ralph knocks over the police officer and flees. Taking aim, the officer fells Ralph with a bullet. Dennis then drives Julia back to the sanity and security of London, and when he proposes that she become his wife, she accepts.