Cerys Thomas is seventeen, grieving for her beloved grandmother and tormented by a terrifying presence that haunts her night-time world. Seeking respite, she flees her London home to spend the summer with her uncle in Quaintlip, an ancient rural village with a sinister history. There, she is befriended by Theresa, the strange and captivating occupant of a seemingly -abandoned cottage on the edge of the village. But as Cerys's nocturnal terrors spiral rapidly out of control, she tries desperately to extricate herself from Theresa's grasp, and finds herself in a battle not only for her own sanity but also for her soul.