With his wife uninterested in fishing, Dr. Paul Martin goes on a holiday on the Cornwall coast alone. There he snags Miranda, a mermaid, and is pulled into the water. She keeps him prisoner in her underwater cavern and only lets him go after he agrees to show her London. He disguises her as an invalid patient in a wheelchair and takes her to his home for a month-long stay.
Martin's wife Clare reluctantly agrees to the arrangement, but gets him to hire someone to look after their guest. He selects Nurse Carey for her eccentric nature and takes her into his confidence. To Paul's relief, Carey is delighted to be working for a mermaid as she always believed they exist.
Miranda's seductive nature earns her the admiration of not only Paul, but also his chauffeur Charles, as well as Nigel, the fiancÃ© of Clare's friend and neighbour Isobel, arousing the jealousy of the women in their lives. Nigel breaks off his engagement, but when he and Charles discover that Miranda has been flirting with both of them, they come to their senses.
Clare finally figures out what sort of creature Miranda really is. Miranda overhears her telling Paul that the public must be told. She wheels herself down to the river and makes her escape.
In the final scene, Miranda is shown on a rock, holding a merbaby on her lap.
What does Clare finally figure out?
What sort of creature Miranda really is