Newlywed American heiress Kit Preston (Doris Day) is living with her financier husband Tony (Rex Harrison) on Grosvenor Square in London. Returning home in a dense fog, she is startled by an eerie, electronic-like voice threatening to kill her. The voice calls her by name and torments her as she runs to escape. Tony tries to convince her she has been the victim of a practical joker and suggests they travel to Venice for the honeymoon they never had.