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One night, when Cheryl Draper wakens and gets out of bed to close her bedroom window, she sees a man, Albert Richter, strangling a young woman to death in an apartment across the street. Cheryl phones the police, but before Lt. Lawrence Mathews and Sgt. Eddie Vincent arrive to interview Richter, he hides the body in an unoccupied apartment next door and pretends to have been asleep. After they find nothing to incriminate Richter, Larry suggests to Cheryl that she may have dreamed it all, but she is adamant that she saw the murder. The next day, as she leaves for her job as an interior decorator at a Beverly Hills store, Cheryl sees Richter loading a large trunk into a station wagon. Later, she buys a small pair of binoculars and, while scanning Richter's apartment, notices the empty one next door. Pretending to be interested in renting the apartment, she asks the resident manager to show it to her and notices scuff marks on the floor. When she asks to view a furnished apartment, the manager shows her Richter's, where she finds and removes a pair of earrings. When she shows her find to Larry, he tells her that Richter has just phoned to report that the earrings had been stolen from his apartment and that they had belonged to his wife, who died in Germany in 1943. Cheryl is still at police headquarters when Richter arrives to claim the earrings, and she directly accuses him of the murder, but he emphatically denies it. As he leaves, Richter tells Larry that he thinks Cheryl is mentally unbalanced. However, Larry, somewhat suspicious, has Richter's background investigated and later is able to tell Cheryl that Richter is a former Nazi, now a writer, and is currently romancing a rich widow. Larry is concerned about Cheryl's preoccupation with what she believes she witnessed and invites her to dinner. Richter sees them drive away and mails a letter to himself. A few hours later, Cheryl reads a newspaper account of the discovery of a woman's body in Griffith Park, but Larry tells her that the police are unable to link it to Richter and begs her to give up her obsession. At night, when Cheryl is working in her apartment, Richter arrives to complain about a letter he has received from her, accusing him of murder, and plants the idea in Cheryl's mind that she is ill. Cheryl then traces Larry and rushes to see him, while Richter returns to her apartment and, wearing gloves, types a letter on her typewriter stating that she saw him commit the murder. After Larry is unable to help her, Cheryl feels that she should perhaps see a doctor. Later, Larry is summoned to Capt. Donnelly's office, where Richter has brought the letters he claims Cheryl has been sending him. Although Larry finds it hard to believe that she wrote the letters, he is ordered to bring Cheryl in. When Donnelly produces Cheryl's typewriter and establishes its use in writing one of the letters, Cheryl becomes hysterical and Donnelly orders her committed to a city hospital. She wakes up there in a ward with three unstable women and is later examined by a psychiatrist, who claims to know her case intimately. She is now convinced that Richter did not commit murder and that she must have written the letters. After Larry arrives with a writ for her release and drives her home, she tells him she thinks they should not see each other again. Later, when Cheryl sees Richter buying several newspapers that reveal the identity of the murdered girl, she begins to realize that when Richter visited her he could have left the door off the latch and reentered to type the letters after she went out. She phones Larry, but he is not receptive to her latest theory. Cheryl then goes to Richter's apartment, where he willingly confesses the crime to her, but reminds her that she has been officially diagnosed as unstable and anything she says will not be believed. When Richter shouts in German, claiming that he alone has the key for which history awaits and that the girl he murdered put in jeopardy the future of the world, Cheryl realizes that he is a totally deranged maniac and flees. At police headquarters, Cheryl tells Larry that Richter confessed to her, but Larry can do nothing due to the lack of evidence. However, Larry decides to trace the murdered girl's background and interviews several of her neighbors, unsuccessfully showing them a photograph of Richter from one of his book jackets. He then decides to check the victim's apartment, where he finds a copy of one of Richter's obscure books. Meanwhile, Cheryl has driven home contemplating suicide to find Richter waiting in her apartment with a suicide note he has written for her on her typewriter. Richter then attempts to throw Cheryl out of a window, but is interrupted by the arrival of a policewoman Larry has assigned to look after Cheryl. Cheryl flees, pursued by both Richter and the policewoman, and runs through the night streets to the construction site of a multi-story building and climbs up to the top, followed by Richter. A crowd gathers and attracts Larry on his way to visit Cheryl, and he and several policemen race up to the top floor. When Richter corners Cheryl, she falls onto a lower platform ledge. After Larry and Richter fight and Richter falls to his death, Larry and others are able to pull Cheryl to safety just as the ledge gives way. Larry and Cheryl embrace, then walk off together.