Northern Irish police officer DCI Tom Brannick connects a suicide note with an infamous cold case with enormous personal significance.

There is something horribly wrong with the bodies found in the dark city streets. Some are mutilated while others have the Price equation (wΔz = Cov (w,z) = βwzVz) carved into their flesh. Detective Eddie Argo and his new partner Helen Westcott unearth the meaning of the odd equation and realise each victim is being offered a gruesome choice: kill your loved ones, or be killed. Before long it becomes clear that the perpetrator has suffered a similar fate and is now coping by seeking a way to solve this philosophical dilemma.