While on holiday, Caroline (Glynis Johns) meets a mermaid named Miranda (also played by Johns) who happens to be her distant relative. Because the two look so similar, they decide to switch places so Miranda can see what life on land is really like. Miranda meets Ronald (Peter Martyn), to whom Caroline is engaged, and she immediately doesn't like him and wants better for Caroline. Looking for suitors, Miranda finds a couple who she thinks are worthy of their bloodline. But Miranda ruffles some feathers and is about to be outed as a mermaid until Caroline swoops in. The question that remains, though, is what becomes of the messy love story Miranda created for Caroline.
