At the edge of the Pennine Moors, Detective Sergeant Blackshaw and his rookie colleague, Detective Constable Lumb, are called to the scene of a double murder committed on a burial ground.

They discover that the case is connected to an ancient Celtic cult, in the remote village of Pen Crags. Desperate for career advancement and obsessed with solving the mystery, DC Lumb immerses himself in the group - and soon disappears.

Amid a chain of unexplained deaths, the Sergeant Blackshaw attempts to unravel the sinister plot. But with scarce evidence, can he prove his theory - or will he have to accept that there is no way to catch a phantom?