It is a worried letter for Tom Yardley, an old army comrade who once saved his life, that takes Inspector Sam Blackstone to the salt mining village of Marston. But by the time he arrives there, Yardley is already dead - blown to pieces in what was apparently a freak accident. Acting strictly unofficially, and without the assistance of his usual team - Ellie Carr is helping to track down a violent serial killer in Staffordshire, Archie Patterson is on the trail of a child prostitution ring in London - Blackstone begins to investigate Yardley's convenient death. It does not take him long to realize that there is true evil lurking in - and under - the village. And before the horrifying truth can finally be revealed, there will be three more deaths, and an attempt on Blackstone's own life.