A young married physician out on a fishing holiday ends up being saved by a mermaid, who intends to keep him her prisoner. She then offers to release him if he will take her to see London, which leads to a number of humorous and romantic entanglements as the mermaid entices several unmarried men who live near the physician and his wife.
A young married physician discovers a mermaid, and gives into her request to be taken to see London. Comedy and romantic entanglements ensue soon after.
- Young physician Paul Martin (Griffith Jones) wants to go on a fishing holiday, but his wife Clare (Googie Withers) is not interested. Paul goes alone off to the Cornwall coast, where his line snares on something, and he goes underwater.
Paul comes to, and finds himself in a cave along with Miranda (Glynis Johns), a beautiful mermaid. Due to the nature of the cave, Paul is unable to leave, and Miranda delightfully explains that she intends to keep him as her prisoner (claiming an aversion to human men). Paul also finds the cave littered with newspapers and magazines, of which Miranda has read about humans and their ways. After a little time, Miranda decides to make Paul a deal: she will let him go if he will take her to London to see the sights. Miranda even concocts the perfect plan to get her about: they'll simply cover up her tail and say that Paul (as he is a physician) is helping her to recover.
Paul is a little incensed at the plan, but goes along with it, ordering several dresses from a fashion designer he and his wife know...though the designer is rather perplexed regarding the need for a hemline that covers past a normal woman's feet.
Paul returns to London with Miranda, but sends her out for a drive to see the city with the house butler, Charles (David Tomlinson). While they are out, Paul explains to Clare that Miranda will be staying with them for 4 weeks, and has arranged for a woman named Carey (Margaret Rutherford) to look after her.
Over the course of the next few weeks, Miranda often displays some rather strange traits: she prefers to only eat raw fish or oysters, and some nights, she can be heard singing a lilting song in a rather unheard-of yet beautiful voice.
She also manages to charm Charles (who is engaged to Betty (Yvonne Owen) the Martins maid), as well as a nearby artist named Nigel (John McCallum) who it just so happens is also engaged. Miranda seems to love the game of enticing the different men around her who seem more than willing to fall at her fins. Miranda even gets to Paul as well, going out for a late-night drive and frolicking together in a watery pond.
Aside from Paul, the only other person who knows is Nurse Carey. Surprisingly, Carey does not freak out, but is delighted, having believed that mermaids existed for many years.
Soon, tensions rise when Charles' and Nigel's fiancées believe that their men are being charmed away from them, with Nigel's fiancée becoming jealous when she sees he's painted a portrait of Miranda.
Clare meanwhile, has been noticing odd things here and there. A large fishbowl that once contained numerous small fish has been becoming increasingly emptier, and reports of Betty finding seaweed in Miranda's bathtub (not to mention finding the girl has no panties at all!), leads Clare to suspect that Miranda is a mermaid.
The whole mess comes to a climax when Charles and Nigel come to see Miranda, both intending to propose marriage to her. It is then they each reveal that Miranda gave them a lock of her hair, and now feel foolish for having been 'played along' by her. Both men then leave to reconcile with the women they have spurned.
Paul arrives soon after with Miranda, which Clare soon gets to admit is a mermaid. After finding that Charles and Nigel returned the locks of hair she gave them, Miranda admits to Clare that she intends to leave soon, intending to swim to warmer climates for the month of May. Miranda goes to her room, claiming to get ready to bathe, and willing to allow Clare to see her tail. However, 5-7 minutes later, Clare and Paul find her room empty, and notice Miranda outside, wheeling her chair to the nearby river. Giving chase, they watch as she dives in, and begins to swim towards the sea.
As Paul wonders why Miranda would choose to go to warmer climates in the month of May, we cut to an image sometime later of Miranda, sitting on a rock holding a little mer-baby (as to who the father of the child is, it is never revealed).