Polly Jenkins is good at school, but really excels at one thing - swimming! She is always top in any swimming competition, and living near the coast in Cornwall is able to swim every day in the sea as she never feels the cold. One day she notices a birthmark on her back shaped like a small fish, and although initially worried about it, feels reassured by her mother's lack of concern. However, shortly after this, at a swimming tournament, she meets a girl called Lisa Roberts and notices the same small fish on Lisa's back. The girls discover that both of them were adopted after having been found on a beach. They also find out they possess an ability to swim underwater for long periods of time...This discovery leads them to a city underwater, a completely different way of life and some startling revelations...