On his first day on a new job DCI Tom Mathias investigates a blood soaked bathroom and the disappearance of 64 year old Helen Jenkins. The inquiry focuses on an old Children's home above the ravine spanned by the Devil's Bridge.

In the isolated village of Penwyllt, the body of a young man is discovered in the murky depths of a quarry lake. But who is he, and was this an accident?

At a remote farmhouse, sixty-nine year old Idris Williams is discovered bludgeoned to death, and the investigation into his murder reveals the mountain’s bloody past.

The body of a young woman is found abandoned but carefully placed in the marshes. This case, in the series finale, pushes DCI Mathias to the edge, both personally and professionally.