"I almost envy you," Olive York's friend told her, on learning that Olive was going to Yonder Key as a nurse for the mentally ill Zoé Croome. "Yonder Key is like living in a shell, with the sound of the sea all around."

But when Olive reached the luxurious, gloomy Croome mansion, where the family had isolated themselves for thirty years, she was struck by a sense of danger and evil in the household. Where did it come from? From Zoé, who seemd gentle, but who hid knives and ropes in her room when not watched? From Zoé's embittered sister Joanna, who didn't want Olive to try to cure Zoé? From the strangely silent servant Ezra?

Alone in the world, Olive has no place else to go. She must stay at Yonder - to help Zoé, and save herself from the unseen danger of YONDER.