Beware what waits in the shadows... With one unexpected email from her estranged best friend, Lucy, Mina Murray's carefully curated life is turned upside down. Suspending her psychiatric practice in London, she returns home to the windswept shores of Wales. Faced with everything she's left behind, she soon discovers that Lucy's symptoms mirror those of her mysterious amnesiac patient hundreds of miles away.

With nothing but an untreatable sickness connecting the two women, and with Lucy's life on the line, Mina finds herself asking questions and being drawn ever-deeper into a web of secrets, missing girls, and the powerful, nameless force at its center--one that has been haunting her for years.

As terrible, ancient truths begin to reveal themselves, Mina prepares to confront her own darkest secrets, and with them, an evil beyond comprehension. Together with a group of smart, savvy women, Mina seizes one last, desperate chance to stop the cycle that began so long ago. But there are dangers to inviting the attentions of what might not be a man, but a monster...