At the turn of the century, in England, a blonde South American fortune hunter named Pedro Ferrara kills his first, second and third wives for their money. Later, in London, middle-class Cecily Harrington wins £50,000 in the Calcutta sweepstakes. Determined to use her money to travel around the world with her roommate, Mavis Wilson, Cecily prepares to let their apartment. One prospective tenant is Manuel Cortez, a dark-haired adventurer. As soon as Manuel learns that Cecily has won the sweepstakes, he begins to court her aggressively. At the same time, Cecily awaits a reunion with her fiancé, Nigel Lawrence, whom she has not seen in two years. Over protests from her aunt Loo-Loo, Cecily breaks her engagement when Nigel arrives, and in a few days, she and Manuel are married. For their honeymoon, Manuel takes Cecily to a secluded cliffside cottage in Devonshire, and asks that she not give out the address. Upon their arrival, Manuel's charming facade begins to crack, revealing his furtive, volatile nature. He insists that his basement "laboratory" is off-limits to Cecily and the house servants, Ethel and Billings, and immediately begins digging a grave in the cellar, where he also keeps black hair dye to maintain his disguise. He methodically notes the date August 31st and time nine o'clock p.m. on his calendar. Soon, Cecily notices that her checkbook is missing, and Manuel tricks her into signing papers that give him control over her money. One day, Cecily sprains her ankle and is tended by local doctor Horace Gribble, who notices that, like him, Manuel has a collection of books on criminology. Dr. Gribble points out to Manuel that an article in a recent journal on wife-killer Pedro Ferrara contained a picture of the killer, which is missing from Manuel's copy of the journal. The article states that Ferrara always murdered his victims at nine o'clock, and reportedly died by drowning, although his body was never found. After a short visit with Mavis and Loo-Loo in London, Cecily returns to the cottage with Manuel to pack for their voyage to the Persian Gulf, India and Baghdad. On August 31st, while Manuel is out making arrangements to have their trunks picked up by a carter, Dr. Gribble drops off his copy of the Ferrara article to prove his point about the picture. Although Manuel had told her earlier that it is bad luck to open a steamship ticket envelope prior to a journey, Cecily opens theirs when she accidentally spills ink on it. Finding the envelope empty, she then opens a letter from a bank, which states that her fortune has been converted into bearer bonds that are now ready for Manuel to pick up in Scotland. Terrified, Cecily finds the key to the basement, and there discovers hair dye, her check book and a box of jewels, which Manuel had collected from other women. Next, Cecily opens the article on Ferrara and is horrified to see Manuel's likeness in the sketch. Meanwhile, Nigel has gone to Scotland Yard to report Cecily's sudden marriage and their secret honeymoon and subsequent voyage, for which he could find no steamship reservations. Inspector Hobday tells him that a murderer matching Ferrara's description recently escaped from South America. Loo-Loo then remembers Cecily's mentioning Dr. Gribble, and Hobday and Nigel search for the location of his practice. As a storm brews, Cecily runs from the cottage just as Manuel returns and forces her into the house. He soon discovers that she knows his true identity, and she bides her time until nine o'clock by making him dinner. When she tries to trick him into believing that she has poisoned his coffee, Manuel starts to strangle her. Nigel and Hobday arrive in time to rescue her, and Hobday pursues Manuel as he flees the cottage, and fights him. As the carter's carriage approaches, Manuel falls into the street and is crushed to death by the horses. Inside the cottage, Cecily embraces Nigel.