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.

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.

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?

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.