Not long out of the fast-track training course at Edinburgh's police college, Detective Sergeant Carla Herron is about to be tested to breaking point. She's been called to Deepwell psychiatric hospital in the Scottish borders to interview a patient who has confessed to the murder of one of the hospital's psychotherapists. The confession is vividly detailed, but for a man locked in a secure ward and under 24-hour surveillance, it is also utterly impossible....