Seraphine Mayes and her twin brother, Danny, were born in the middle of summer 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 who drew a young nanny into their inner circle.

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. It was taken on the day the twins were born, and in the photo, their mother, surrounded by her husband and her young son, is smiling serenely and holding just one baby.

Who is the child, and what really happened that day?