One December night, in a seaside town, a little girl is found abandoned. When her mother finally arrives, authorities conclude that it's a case of a toddler running off. Even as DS Joanna Harper investigates an assault on a man named Gregor, she sees footage of the mother-daughter pair from town, and realizes that the woman is her estranged daughter, Ruby. Somehow Ruby is connected to Gregor's family—including his wife, who believes she is descended from the mythical Selkies. It's already a strange case, but Harper realizes she may have to choose between Ruby and her career.