Gymnastics school teacher Caroline Trewella (Glynis Johns) inherits a house on the Cornwall coast. In the basement/smuggler's cave, she discovers mermaid Miranda (also played by Glynis Johns). The young women share a common ancestor and are remarkably alike in looks. While Caroline is away on a bicycling tour, Miranda has fun taking her place on land. With the help of her friend Nurse Carey (Margaret Rutherford), Miranda feigns an injury that keeps her wheelchair-bound - or more often needing to be carried by a man. And the ever flirtatious Miranda certainly knows how to attract all of the men of the village.