As the Lyell team focus their investigation, they bring peril upon themselves. With the world they hold dear teetering on a knife edge, they confront their threats to preserve the credibility of DNA.

The death of an investigative journalist throws the spotlight onto Sam Ryan. Nikki begins to realise that the implications of her discoveries will challenge the principles of forensic science.

A secret from his father propels Jack to Belfast. Meanwhile, Simone and Nikki investigate the mystery of a burned body. As familial secrets come to the surface, Jack faces tragedy head on.

When a woman confesses to murdering her husband, the Lyell team must determine if all is as it seems. Meanwhile, Simone is shocked to be reunited with a detective from her childhood, and struggles to untangle her past.

DNA evidence linked to the Health Secretary's assassination forces Nikki to face her past in the present. As her investigation progresses, Nikki realises that its consequences will affect her, and her profession, forever.

In Liverpool, following the launch of Sam Ryan's new company, the health secretary is assassinated. Sam summons the Lyell team with a plea to help investigate, but Nikki is unsure whether she can trust Sam, who seems to know more than she's revealing. Meanwhile, Jack and Nikki navigate their feelings for each other.