Sanna is a mermaid — but she is only half seavish. The night of her birth, a sea-witch cast a spell that made Sanna’s people, including her landish mother, forget how and where she was born. Now Sanna is sixteen and an outsider in the seavish matriarchy, and she is determined to find her mother and learn who she is. She apprentices herself to the witch to learn the magic of making and unmaking, and with a new pair of legs and a quest to complete for her teacher, she follows a clue that leads her ashore on the Thirty-Seven Dark Islands. There, as her fellow mermaids wait in the sea, Sanna stumbles into a wall of white roses thirsty for blood, a hardscrabble people hungry for miracles, and a baroness who will do anything to live forever.