After Saoirse discovers her selkie coat on the night of her birthday (when dad is at the pub unhappy about losing his wife, and granny is taking care of the 2 children), she goes out to sea and plays with her seal friends. Granny discovers her washed up on the beach and forces Dad to let her take the children away to the city. Neither of the children want to go, and after they are there only a couple hours, they run away, trying to get back to their dad and the lighthouse that is home. The film is the journey that takes them from Granny's home in the city back to their dad's place. Along the way, Ben discovers that his sister is one of the Sidhe (pronounced "she"), the fairy-folk, and has to rescue her from the claws of Macha, the owl goddess.