Engineer Bruce Templeton (Rod Taylor) makes quite a catch when he reels in public-relations worker and part-time mermaid Jennifer Nelson (Doris Day). Smitten with the costumed cutie, Bruce commissions Jennifer to write his biography. But through a series of misunderstandings, he comes to believe she's a Russian spy intent on stealing scientific secrets. A comic pleasure cruise, this lighthearted romance also features some great musical numbers.