Seraphine Mayes and her twin brother, Danny, were born at their family's estate on the Norfolk coast. Within hours of their birth, their mother threw herself from the cliffs, the au pair fled, and the village thrilled with whispers of dark cloaks, changelings, and the aloof couple and their young nanny. Now an adult, Seraphine mourns the recent death of her father. While going through his belongings, she uncovers a family photograph that raises dangerous questions. Taken on the day the twins were born, it shows her mother smiling serenely and holding just one baby. Who is the child, and what really happened that day?