When gymnastics school teacher Caroline (Glynis Johns) goes on holiday at her family's home in Cornwall, she meets her distant mermaid relative, Miranda, who looks exactly like her. Caroline allows Miranda to trade places with her, while she embarks on a bicycling trip with a friend. To facilitate the ruse, Caroline pretends to be injured and confined to a wheelchair, enlisting Nurse Carey to attend to Miranda and conceal her mermaid secret. Caroline is engaged to Ronald Baker (Peter Martyn), but Miranda quickly dislikes him after discovering his poor environmental ethics. Miranda sets out to find a better match for Caroline, flirting with bachelors Jeff Saunders (Donald Sinden) and Colonel Barclay Sutton (Nicholas Phipps), much to Ronald's dismay. Meanwhile, Barbara Davenport (Anne Crawford), the colonel’s fiancée, becomes suspicious of Miranda and plots to reveal her secret by arranging for 'Caroline' to sing at a charity concert. Realizing Barbara's plan, Caroline returns just in time to reclaim her place and protect Miranda. The story concludes with romance as Jeff takes Caroline boating, leading to a heartfelt moment observed by a wistful Miranda. Cast: - Glynis Johns as Caroline Trewella / Miranda Trewella - Donald Sinden as Jeff Saunders - Anne Crawford as Barbara Davenport - Margaret Rutherford as Nurse Carey - Dora Bryan as Berengaria - Nicholas Phipps as Colonel Barclay Sutton - Peter Martyn as Ronald Baker - Noell Purcell as Percy - Joan Hickson as Mrs. Forster - Judith Furse as Viola - Irene Handl as Mme. Blanche - David Hurst as Signor Mantalini - Martin Miller as Dr. Fergus - Deryck Guyler as Editor - Anthony Oliver as Pawnbroker Run Time: 90 minutes.