In a small Irish fishing village, a 10-year-old girl named Fiona is sent to live with her grandparents, where she discovers a local legend about her ancestor marrying a Selkie, a seal with the ability to transform into a human. As she settles into her new home, Fiona learns that her baby brother mysteriously disappeared at sea years ago, with some family members believing he was taken in by the seals. When Fiona spots a naked young boy on a deserted island, she takes it upon herself to uncover the truth about the island and the boy's identity.