The wife of an Irish school teacher is branded a traitor when she falls for a British officer who is part of an occupying force in 1917 Ireland.

Ten-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 was washed out to sea and never seen again, so when Fiona spies a naked little boy on the abandoned Isle of Roan Inish, she is compelled to investigate.