When Leo's friend commits suicide, Harry and Nikki are called to the scene of a violent and calculated murder, which is thought to be the work of an apparently dormant serial killer.
Harry and DI James deviate from the original investigation to secretly test DNA evidence from the rival Chesham laboratory to try to solve this stalled case.
With Harry dead and Leo and Nikki getting nowhere with the police, the pathologists struggle to clear Harry's name, while the police seem intent on covering up a wider conspiracy.
Having established that Lieutenant Lockford was murdered, Leo makes a startling breakthrough, and the investigation begins to unmask a dark secret at Hillsdon Army Base.
Leo investigates a potential copycat killing, but the team are starting to wonder if Karl Bentley was wrongly convicted, leaving the real murderer free to set out on a new killing spree.
Nikki's mental instability worsens, and Harry and Leo are forced to get her psychiatric help. But then when the true identity of the murderer is discovered, it becomes a race against time to save Nikki's life.
