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.
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.
When the Van Burens' au pair commits suicide, the pathologists start to unravel the truth behind the embassy shootings, and a dark family secret is exposed.
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.
When human rights lawyer Anna Sandor calls Harry to Budapest to investigate the death of a client, they start to uncover a sinister underworld conspiracy, putting both their lives in danger.