Alison Willetts is unlucky to be alive. She has survived a stroke, deliberately induced by a skilful manipulation of pressure points on the head and neck. She can see, hear and feel and is aware of everything going on around her, but is completely unable to move or communicate. Her condition is called Locked-In Syndrome.

In leaving Alison Willetts alive, the police believe the killer made his first mistake.

Then D.I. Tom Thorne discovers the horrifying truth; it isn't Alison who is the mistake, it's the three women already dead. "An appropriate margin of error" is how their killer dismisses them, and Thorne knows they are unlikely to be the last. For the killer is smart, and he's getting his kicks out of toying with Thorne as much as he is pursuing his sick fantasy. Thorne knows immediately he's not going to catch the killer with simple procedure. But with little more than gut instinct and circumstantial evidence to damn his chief suspect, anesthetist Jeremy Bishop, his pursuit of him is soon bordering on the unprofessional. Especially considering his involvement with Anne Coburn, Alison's doctor and Jeremy's close friend.