When the pathologists investigate three suspicious deaths in one night on the same ward, it looks like hospital staff may be behind their murders. Nikki becomes increasingly disturbed by a rape and murder case involving a young child.

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.

Leo finds himself haunted by past memories when he returns to Sheffield after an ancient body is unearthed on Bleaklow Moor.

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.

As Nikki investigates the drowning of a young mother, Harry is called to an apparent suicide at a nearby army base; it soon starts to look like the cases may be linked.

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.

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.

After a major incident at the Dutch Embassy, Harry and Nikki are removed from the case when the Dutch decide to appoint their own pathologist.

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.