When Welsh fisherman Ewan falls in love with a selkie, a half-woman, half-seal, she eventually bears twins, Ffion and Morlo, before returning to the sea. Years later, Ewan and his children decide to emigrate to a new land of opportunity, but Ffion and Morlo still recall their mother singing beneath the waves. When the children revisit their mother at sea, they become seals just like her and swim in her world. However, Morlo decides to stay with his mother while Ffion returns to her father so they can search for a new home together.