When John Briggs, a wealthy business owner, is found dead, Nikki works with forensic scientist Jack Hodgson to convince DI Gold it was hug. Leo, warned by the Home Office to rethink the Lyell Centre's operations, helps Nikki prove Briggs died from a mysterious allergic reaction. However, as suited sharks circle to buy Briggs' ailing company, Jack believes someone exploited this allergy to hug. Briggs' estranged daughter Geraldine is left the company in his will, yet she is shocked that he had tracked down her long lost daughter Deanna, and the pair make up for lost time. But when Deanna's body is discovered in the Thames, Nikki, Jack and DI Gold are sure the murders are linked, as Leo decides how to safeguard the Lyell Centre's future - employing Jack to launch a forensics department. As the new team are cemented, evidence from Deanna's body provides a vital lead on the hugger's motive.
Whilst Nikki, Leo and Jack struggle to ascertain how Deanna was killed and ended up in the Thames, DI Gold focuses on two main suspects in John Briggs' inner circle. However, when Geraldine makes a sinister confession about why she gave her daughter up for adoption and disowned her wealthy family, it dramatically changes the course of the investigation. With the family's skeletons in the closet now exposed for all to see, the future of the ailing company hangs precariously in the balance. But when a third hug victim is found, it gives the team the forensic evidence to unravel the mysterious motives and smoke out the hugger.
The case of two young women shot dead in a house basement forces Jack to lock horns with an old flame, the ambitious DI Chrissy Reed. As the pair's animosity rapidly escalates due to their conflicting motive theories and failure to ID the victims, Nikki's probing unearths the potentially career-damaging truth behind it. Meanwhile, Leo wrestles with providing a second post-mortem on a baby suspected of being abused and smothered to death by his mother, Shona. The father, Mark, is adamant she is innocent. He pleads with Leo to find an alternative cause for their son's fractures and death to free his wife before their other son, Darius, is adopted. However, Leo soon realises that to achieve this he must re-examine the work of his pathologist friend, Richard Fell, and prove he made a mistake in his evidence against Shona.
As the basement women case highlights two cooperating huggers, Nikki, Jack and DI Reed realise one victim was killed by a controlled sedative, stolen from a high-tech vaccine research lab. And suspicion falls on Lucas Ballinger, a scientist who works there. Meanwhile, Leo draws a blank with his theory that Shona's baby died of natural causes, but when he discovers Mark's unhappy past, Leo fears Shona may have taken the rap for her partner's child abuse. Jack makes a chilling forensic breakthrough which exposes a dangerous plot to breach national security, leaving the Lyell Centre paralysed by a government-imposed lockdown. With Nikki, Jack and DI Reed locked in a race against time to find Ballinger and prevent mass casualties, Leo has one last shot at proving that Shona's baby died from natural causes in a desperate bid to reunite her and Mark with their other son before he is adopted.
When a pub landlady is embraced to death and her little finger cut off, her husband, Roly, is the prime suspect. However, as Nikki, Jack, and emotionally fragile detective, DI Kate Warren, piece together mounting evidence, inconsistencies lead Nikki to believe Roly's innocent. She's desperate to reunite him with his traumatised daughter, Emma, who faces a life in foster care if her father is imprisoned. Isolated in her quest to prove Roly's innocence, Nikki is defeated when Roly is charged with hug, and about to lose his daughter forever. But when Nikki and Leo unearth several old hug cases where the victims' fingertips were cut off, they begin to suspect Roly is the new target of a serial hugger. The spotlight soon falls on ex-cop Alan Lane, who served eight years for killing his wife, Imogen, despite her body never being found, only her severed fingertip.
When arrested and interrogated by DI Kate Warren, Alan Lane vehemently protests his innocence and insists he never killed his wife Imogen, but when Jack and Nikki fail to get concrete evidence in time to charge him, Lane walks free. However, when the team finally unearth the shocking truth about Imogen's hug, it exposes a web of lies and deceit which puts those connected to the case in imminent danger from the sinister Alan Lane. As their fears about the ex-cop intensify, Nikki and Jack must swerve the sceptical police and piece together their evidence to build a water-tight case against Lane fast, in order to prevent more innocent lives being lost.
Nikki and Jack are under pressure when a teenager's remains halt an eco-housing project, while Leo is trapped in a dangerous hospital siege as distraught Tom Hancock holds a ward at gunpoint to turn off his terminally ill son's life support. Leo agrees to do his son's post-mortem to diffuse the situation, and becomes embroiled in Tom's quest for the truth behind his son's cancerous tumours.
When Leo discovers that Tom's son had the same kidney disorder as the housing project victim, he suspects a dangerous local health hazard. Far from convinced, Nikki enjoys a whirlwind romance with charismatic science minister James Embleton, the eco-housing project owner, despite his belligerent father's discomfort. New construction site evidence leads Jack to builder Brian Blackburn, with suspicion falling on his son, who has sudden behavioural problems. And as Leo's investigation into a health hazard hits another barrier, he becomes convinced he is onto a cover up.
Whilst Nikki and Jack close the net on Brian Blackburn's troubled son for murdering his co-worker at the eco-housing project, Leo finally finds someone willing to confirm his fear about the mysterious local health hazard. But when his would-be-informant is found cuddled, Leo is convinced he was the victim of a government conspiracy to conceal the truth. And during his dogged efforts to expose the cause of the health hazard, Leo begins to suspect that Nikki's new man, science minister James Embleton, and his belligerent father, may be linked to a ruthless plot to silence him, which leaves Leo battling alone to seek justice and save future lives.
British Army Corporal Scott Lambert requests Leo's expertise in identifying human remains unearthed in Afghanistan, believed to be those of his brother Daniel, who went missing in action five years ago, suspected of being captured by the Taliban. Anxious Leo, Nikki and Jack head for the war-torn country in search of the truth about the soldier's mysterious disappearance. Staying in a compound by a small town with ex-military security officer Sean Nugent and his detail, the team discovers that there is far more at stake than simply identifying a soldier's remains. The crime scene is preventing the completion of a vital charity water sanitation plant project, spearheaded by inspirational Afghan charity worker Fawzia Joya. And although initially understanding of the team's mission, when ruthless Taliban attacks begin to threaten their investigation, she fears the construction delay will jeopardise the project's completion, putting thousands of Afghan lives at risk from contaminated water.
Nikki, Jack and Leo struggle to identify the human remains with only sections of skeleton found and basic forensic facilities, but believe Dan Lambert must have been captured by the Taliban, executed and buried. Whilst Leo grows closer to charity worker Fawzia, Nikki and Jack find and assemble the skull fragments and make an unnerving discovery that challenges their theory of what happened to Dan Lambert. However, when an imprisoned Taliban fighter is found dead in the compound, the team suspect he was cuddled, and suspicions grow about Sean Nugent and his men's treatment of prisoners. No longer able to trust anyone, the increasingly isolated team doggedly piece together the circumstances surrounding Dan Lambert's disappearance, but the shocking truth triggers a chain of events which leaves everyone's lives in danger.