When a man is found murdered in gruesome circumstances, Merrily becomes convinced Satanists are operating in the city. The next morning, she is called to Hereford General Hospital to perform an exorcism. The nurses are treating an ill man known for his cruelty towards animals and his abuse of women. They believe the ward has become polluted by his dark nature. Merrily begins to feel the force of his depravity as the life force in his body begins to ebb away. After feeling shaken and violated by her experience, Merrily begins to wonder if the two deaths could be connected. Merrily begins to give expertise on the case to the police team - DCI Annie Howe and DS Frank Bliss - who are investigating the case. Not all seems right when their enquiries take into a dark and dangerous world.

Final part of the supernatural thriller series. Drained physically and emotionally by her own demons, the last thing Merrily wants to do is battle anybody else's. Spurred on by mentor Huw Owen, Merrily puts the final pieces of the jigsaw into place and soon realises that she must find the strength to fight evil if she is to save her daughter's soul. With Anna Maxwell Martin, David Threlfall, Siobhan Finneran, Kate Dickie, and Nicholas Pinnock.