DCI Mathias’ investigation into the disappearance of 64 year old Helen Jenkins leads him to the cascading waters of an ancient ravine at Devil’s Bridge, and uncovers the cruel history of a long closed children’s home.

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.