A private jet crashes into woodland outside London. Its passengers include the former US ambassador, Jonathan Kraft. Nikki calls her partner Matt from the crash scene to break the news - his friend and former colleague did not survive the accident. Matt is desperate for answers. When Thomas is called to an apparent suicide of a successful businessman, he finds unexpected connections to the plane crash. Are they dealing with a coordinated attack?

Nikki and the Lyell team continue to investigate the plane crash, ruling out an explosion and engine failure. Suspicion turns to Rowan Cole, the so-called Deadhead - a non-flying pilot who was hitching a lift back when his intended flight was cancelled. The investigation takes a turn when they discover a connection to a sinister online forum. Nikki’s partner Matt is grieving following the death of his close friend, putting their relationship under strain.

When a body is found in a field near the village of Hartford, Nikki and the Lyell team confirm it is that of missing teenager Jason Forbes. Working to the theory that most killers strike close to home, the ambitious DCI Claire Ashby is committed to proving the guilt of her main suspect, local locksmith Malcolm Wilde.

Investigating the murder of a teenage boy, Nikki and her team are forced to question police methods. Although Thomas discovers evidence that throws the time of death into doubt, DCI Claire Ashby seems more concerned with building the case against her main suspect. With pressure building on the police to secure a conviction, the Lyell team must fight to ensure that inconvenient truths aren't overlooked. Meanwhile, Clarissa notes a troubling parallel with a case from the start of her career. Is the real murderer still out there?

The body of a woman is found on railway tracks in London, badly damaged from electrical burns. On the mortuary slab, the woman’s recent and historic injuries suggest years of abuse, likely within the home. It’s a difficult post-mortem for Nikki, as she is troubled by memories from her childhood.

Nikki determines that Jenny must have died before making contact with the tracks. But when the search for Jenny’s killer hits a wall, her friends at the women’s refuge seek answers. Meanwhile, Thomas’s political ambitions lead him to make friends in high places. Offers are made to open all the right doors for Thomas, but what do they want in return?

When a skeleton is found in concrete, the Lyell team must investigate a suspicious death dating back 20 years.

The team's investigation leads them to a mysterious cryogenics facility and the discovery of another suspicious death.

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 aneurysm, but when Thomas discovers the trace of a possible nerve agent in his system, the family are desperate for answers.

Jack is rushed to hospital after collapsing at the scene of a car crash. Thomas discovers that soldier John Sealy may have been exposed to a nerve agent.