A killer hides deep in the darkness… 

DI Rob Marshall is stuck in Gartcosh, leading the new Behavioural Sciences Unit – a staff of one and a third – where he’s executing a fruitless hunt, profiling every single murder in Scotland just in case they’re serial cases.  

But when a young Peebles woman’s body is discovered in Neidpath Tunnel, disused since the sixties, Marshall is called in to consult on a case on his old home turf. Paired up with his replacement, a cop who’s disinterested in the work, Marshall must tread a line between helping and not jumping to conclusions. 

As the scope of the case wides, can Marshall find a killer who may have led many down his path of darkness? Or is Marshall on his own road to destruction?