Ben lives in a lighthouse with his father and mother, who enchant him with tales of selkies who transform from seals into humans. His happiness is disrupted when she passes away in childbirth leaving behind an unusual little sister, Saoirse, who remains speechless until her sixth birthday. Their father, still grieving, sends them to Dublin to live with their grandmother, separating Saoirse from the ocean that is her life-force and Ben from his beloved hound, Cú. They set off home on a perilous journey encountering mythical demons along the way