When gymnastics school teacher Caroline goes on holiday at her family's home in Cornwall, she meets her distant mermaid relative Miranda, who looks exactly like her. She agrees to let Miranda trade places with her, while she goes on a bicycling trip with a friend. Caroline feigns an accident, pretending that this requires her to use a wheelchair for a few weeks, thus providing a cover for the fact that Miranda has a fish tail instead of legs. Nurse Carey, who knows about Miranda [and was in the earlier film], is hired to attend Miranda.
Caroline is engaged to Ronald Baker, but when he shows up, Miranda does not like him at all. She decides to make Caroline a better match. She flirts outrageously with two eligible bachelors, Jeff Saunders and Colonel Barclay Sutton, right in front of Ronald. When she discovers that Ronald works in the government sanitation department (and approves of dumping garbage into the ocean), she dumps a tureen of cold soup on his head.
Meanwhile, Barbara Davenport, the colonel's fiancée, takes an understandable dislike to Miranda. While out swimming, she discovers Miranda's secret and arranges for "Caroline" to sing at a charity concert, plotting to reveal her true nature. Caroline reads about the forthcoming concert during her holiday, guesses what Barbara intends, and rushes back to take Miranda's place, foiling Barbara's scheme.
Afterward, Jeff takes Caroline boating. When he tries to kiss her, she resists at first, then willingly gives in, while a somewhat sad Miranda watches.
![](./File:"Mad_About_Men".jpg){kind=link}