An unknown and dangerous threat is lingering on the fells. DI Sam Cobbs and her team are called in to investigate the disappearance of a couple of experienced hill walkers. Vanished without a trace. When another couple go missing in the same location, Sam realises she and her team need to quickly uncover the motive behind the complex case, a case where Sam is forced to make some dubious decisions, which could be about to put the walkers at greater risk.