Clarrisa's husband Max is brought to the Lyell Centre to help crack a case.

Becky's father Stuart is arrested following the discovery of footage on Frankie's phone of Becky having reluctant sex with both the dead boys. However Clarissa spots Becky's devoted flat-mate Paul watching them surreptitiously though his DNA is not found at either murder site and he has an alibi for both crimes. Then headmaster Clive Maitland is stabbed to death, following a run-in with two ex-pupils though his murder seems unrelated to the others. When Max is recalled he establishes a lethal link between two parties with no obvious link.

Pregnant Karen Sawyers is murdered and her blood-stained husband Pete found wandering the streets but DI Naomi Silva turns her attention to Karen's lover, Adam Hayes, who was treating her at his clinic and had arranged for her to have a termination. Then Adam is also found dead. Clarissa finds evidence that Karen was being blackmailed with threats of Pete getting her medical records. This leads Naomi and Nikki to hospital head Simon Laing, who tells them a computer hacker calling himself Splinter stole patients' files though Simon kept it quiet for fear of tarnishing the hospital's reputation. Meanwhile barrister Jason Farrell is viciously attacked and left to die.

With Jason in a coma the police, given his hospital attendance, assume he was Splinter's latest victim though he was not robbed. A known hacker Gary Brockham is arrested and admits to selling hospital files on the dark web whilst Laing must admit his use of a questionable computer system company. He too is killed before a secret from the past resolves the case.

On Christmas day the team is called to the home of haulage company owner Andy McMorris to find three corpses and Andy disappeared. Sniper shots ring out, wounding DI Gibbs and McMorris's blooded daughter Mel tells Nikki her father shot at her and her horse. A large number of weapons are found in the home suggesting illegal gun importation whilst the diary of Mel's sister Stevie expresses hatred for her father and his estranged wife Zoe. DCI Cooke, a friend of the family assumes the case but appears to be hiding something and Clarissa discovers that the 999 call reporting the shots was made over twenty miles away.

As the three corpses are identified Vernon Rye, a friend of Andy's father Martin, one of the victims, is pulled in after admitting to a fight with him but Mel, recovering in hospital, tells Zoe and Nikki she heard Andy and Martin rowing before shots rang out. Zoe's sister Jackie is another suspect having had an affair with Andy, but claims she left the house before the murders. The discovery of a missing lorry full of guns and another body proves Andy's criminal activities and reveals who made the phone call. Cooke thinks he has found the killer but the pathologists' evidence proves otherwise.

The team must painstakingly piece together the evidence and unearth the connections between a diverse range of suspects.

Nikki is humiliated in court when a barrister accuses her of making a serious error in the case of a policeman who may or may not have been murdered.

When a skeleton is found buried in cement, the team have to work out how and why he was found there. Meanwhile, Clarissa receives some devastating new about her mother.

As the team looks further into Hope's death, they find ever more questions and realise more crimes were committed decades ago that need resolving. Clarissa has some difficult choices to make.

Thomas investigates the case of John Sealy, a soldier who died during a military training exercise involving exposure to CS gas. It appears John died from an aneurism, but when Thomas discovers the trace of a possible nerve agent in his system, the family are desperate for answers. Was a chemical weapon involved or is there a more innocent explanation?

It appears John died from an aneurism, but when Thomas discovers the trace of a possible nerve agent in his system, the family are desperate for answers. Was a chemical weapon involved or is there a more innocent explanation?