Suzie Gray is only fifteen when she is stabbed to death within metres of her uncle's yacht on the Isle of Wight. Her body is found in the blood-smeared arms of Olly Matken, a family friend who grew up with her. Schizophrenic and vulnerable, he presents a serious challenge to the police. 'I didn't hurt her!' Olly protests. 'All I did was keep her from the devil.' DCI Charlie Trench turns to forensic psychologist Karen Taylor. She knows she should ignore his call, but she cannot. Curiosity and, although she would never admit it to her partner, Will, a dangerous attraction to the brooding detective, push her into a deeply troubling case. Is Olly capable of murder? His own psychologist doesn't think so, but his father does. The only way to find the truth is to identify Olly's devil. And Karen has demons of her own.