As his wife Clare is uninterested in fishing, Dr. Paul Martin goes on a holiday on the
Cornwall coast without her. There, Miranda Trewella, a
mermaid, catches him by pulling on his fishing line and making him fall in the water. She drags him down to her underwater cavern where she keeps him prisoner for a week and only lets him go after he agrees to show her
London, where he lives. Having ordered several extra long dresses from Clare's London
couturier to cover her tail, he disguises her as an invalid patient and takes her to his home, initially for a three weeks stay. Clare reluctantly agrees to the arrangement, but insists Paul hire someone to look after the "patient". He selects Nurse Carey for her eccentric nature who provides a
bath chair and takes her into his confidence. To Paul's relief, Carey is delighted to be working for a mermaid as she has 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 fiancé of Clare's friend and neighbour Isobel, arousing the jealousy of the women. Nigel even breaks off his engagement, but when he and Charles discover that Miranda has been flirting with both of them, they come to their senses. With Clare strongly suspecting that Miranda is a mermaid, she makes Paul admit it. But when Miranda overhears Clare telling Paul that the public must be told, Miranda wheels herself down to the
Thames and makes her escape into the water. She had previously told them she would go to
Mallorca for a visit. In the final scene, Miranda is shown on a rock, holding a merbaby on her lap. ==Cast==