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 shaped like a boat; someone in the family believes the boy is being raised by the seals. Then Fiona catches sight of a naked little boy on the abandoned Isle of Roan Inish and takes an active role in uncovering the secret of Roan Inish.