10 year old Fiona is sent to live with her grandparents in a small fishing village in Donegal, Ireland. She soon learns the local legend that an ancestor of hers married a selkie -- a seal who can turn into a human. Years earlier, her baby brother washed out to sea in a cradle, and some think that he is being raised by the seals. Then Fiona catches sight of a naked little boy on the abandoned isle of Roan Inish, and takes a more active role in uncovering the mysteries which abound.