While out fishing, Dr. Paul Martin is himself caught by a mermaid by the name of Miranda.  She brings him to her undersea cave, agreeing to release him only in exchange for a trip to London as his "patient."  It is made clear early on that Miranda and men find each other irresistible, and she quickly adds the chauffeur and the fiancé of Paul's neighbor to her list of conquests.  This causes more than a little consternation among their respective women, whose pity turns to jealous anger over their eagerness to carry and otherwise indulge the wheelchair-bound Miranda.  Since Miranda sleeps in a tub of cold water at night, and her diet consists almost exclusively of raw fish, it's only a matter of time before her secret is revealed (one hilarious clue is that she treats the goldfish bowl as sort of a candy dish).

Glynis Johns is utterly charming as Miranda, and Googie Withers is excellent as Paul's ultimately understanding wife.  Margaret Rutherford sparkles in a relatively small role—but at least she gets to dance.  Despite the improbable premise, one can't help but be drawn into this very funny film.  And don't miss the scene at the very end, which may leave more questions asked than answered.