As a kid, Jacob formed a special bond with his grandfather over his bizarre tales and photos of levitating girls and invisible boys. Now at 16, he is reeling from the old man's unexpected death. Then Jacob is given a mysterious letter that propels him on a journey to the remote Welsh island where his grandfather grew up. There, he finds the children from the photographs--alive and well--despite the islanders’ assertion that all were killed decades ago. As Jacob begins to unravel more about his grandfather’s childhood, he suspects he is being trailed by a monster only he can see.