In a small coastal village in Ireland, a young boy named Ben lives with his father Conor and his silent little sister Saoirse, who has yet to speak a word. Their mother vanished on the night Saoirse was born, leaving the family enveloped in a quiet sorrow. Ben cherishes a seashell gifted to him by his mother, from which he hears the lullaby that she once sang. Unbeknownst to Ben, his sister is a selkie, a mythological being capable of transforming from a seal into a human. On her sixth birthday, Saoirse discovers a white sealskin coat, which leads her to a transformative experience in the ocean, revealing her selkie nature and a path to a hidden supernatural world.

As the siblings embark on an extraordinary journey, they encounter a variety of magical creatures from Irish folklore including fairies, giants, and spirits. The children's adventure becomes a quest to fulfill their mother's destiny. Along the way, they must contend with the Macha, an owl witch who has her own tragic reasons for wanting to remove all emotions from the land and its inhabitants. Ben, who initially resents his sister for their mother's disappearance, must come to terms with his feelings and learn the importance of family, love, and the deep-rooted cultural legacy represented by Saoirse's identity as a selkie.