It's rare for a young woman to die from a stroke and when three such deaths occur in short order it starts to look like an epidemic. Then a sharp pathologist notices traces of benzodiazepine in one of the victim's blood samples and just traceable damage to the ligaments in her neck, and their cause of death is changed from 'natural' to murder. The police aren't making much progress in their hunt for the killer until he appears to make a mistake: Alison...

Detective Inspector Tom Thorne tracks two serial killers who seem to be working hand-in-hand, trying to catch them before those closest to him get caught in the crossfire of the investigation.