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A taxi drops Marie Morgan off at her deceased grandfather's remote home, which has been shut up for thirteen years. When Marie goes into the dark house, she recalls an important dinner attended by all her relatives thirteen years before, at which her dying grandfather announced that everyone would have to wait thirteen years, until Marie's twenty-first birthday, to learn about their inheritance. Her grandfather then showed her a sealed envelope and told her she would receive it from lawyer Barksdale on her twenty-first birthday. Marie also recalls that the thirteenth chair at the table was empty. She now opens the envelope and finds a slip of paper with the numbers 13-13-13 on it. Someone else then comes into the dark house and a gunshot is heard, after which Marie screams. The taxi driver summons the police, and they find Marie's dead body seated at the dinner table. Later, Marie's uncle Adam, who is unaware of her death, hires private investigator Johnny Smith to protect her, as he distrusts the family, and Johnny arrives at the mansion in time to learn that she was killed by electrocution. Police chief Burke sends his detective, Speed Dugan, to investigate the hotel at which Marie was staying, and Johnny lends him his car after Burke's disappears. Later, Jim, a district attorney, Burke and John interview Marie's brother Harold and her cousin Tom Jackson. After Johnny asks them to draw a diagram of the seating arrangement for the dinner from thirteen years ago, Speed arrives and announces that Barksdale was staying at the same hotel as Marie. That night, Barksdale slips into the house after Speed falls asleep while on guard duty, but when Barksdale answers a ringing phone, a masked man in a secret room pulls a lever and Barksdale is electrocuted. Speed runs from the house in terror, and when he returns with Burke and Johnny, they find Barksdale's body seated at the dinner table in the same spot he occupied thirteen years earlier. Everyone is startled when a very alive Marie is then brought in by a policeman, but Johnny reveals that he had found plastic surgery scars on the first Marie, and theorizes that she was an impostor. Johnny takes Marie to his apartment and there postulates that Adam is the next in line to be killed. After gathering all the Morgans at his apartment, Johnny discovers that they all hate each other, and for their own safety, has them held in jail as material witnesses. Later that night, Johnny is awakened by a call from Burke, who reports another murder in the house. Johnny has the Morgans released and followed, and goes to the house, but learns that the dead body has not been identified. Johnny then relates Marie's story: After she heard the gunshot, she ran upstairs and hid, and when she came back down, she was terrified to find her double dead in the dining room. She stole Burke's car, but was picked up by the police. Although Johnny advises Marie not to tell anyone what was written on the slip of paper, she tells Harold and Jackson. Plastic surgeon Dr. Sherman then comes forward and identifies Jackson as the man on whom he performed plastic surgery. When Johnny learns that someone claiming to be him instructed Marie to go to the Morgan house, he races there with Harold, but Marie has already been kidnapped by a masked man. Jackson confesses to Johnny and the police about his plastic surgery, and that Barksdale was in on the ruse with him. Everyone hears Marie's screams from behind a wall, and the police axe through a bookshelf into a secret room, where the masked man is apprehended and revealed to be Adam. After Adam's arrest, Burke gloats that John was actually hired by the killer, but Johnny claims he knew about Adam from the beginning, and proves that 13-13-13 is the number of grandfather Morgan's bank account. Grandfather Morgan's letter reveals that he has left the entire inheritance to Marie, and that he had hoped that most of the family would kill each other off by the time she came due for it. Having fallen in love, Johnny and Marie then enjoy a kiss.