It's Christmas, and a serial killer is leaving displayed body parts all over Cumbria. A strange message is left at each scene: #BSC6.

Called in to investigate, the National Crime Agency's Washington Poe and Tilly Bradshaw are faced with a case that makes no sense. Why were some victims anaesthetised, while others died in appalling agony? Why is their only suspect denying what they can irrefutably prove but admitting to things they weren't even aware of? And why did the victims all take the same two weeks off work three years earlier?

And when a disgraced FBI Agent gets in touch, things take an even darker turn. Because she doesn't think Poe is dealing with a serial killer at all; she thinks he's dealing with someone far, far worse - a man who calls himself the Curator.

And nothing will ever be the same again....