When Thomas sides against her findings on a controversial case, Nikki suspects foul play and takes steps to prove her theory. Jack and Clarissa follow a forensic trail connecting a dead researcher with a billionaire scientist and a secret testing programme.

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.

Meanwhile the team investigates the death of a medical researcher found in a canal with mysterious puncture marks on his body.

During the team's search for the killer, Jack works alongside an old friend from Northern Ireland, a fearless journalist who has dedicated her life to exposing the suspected organisation. As the team investigate further, they realise they have uncovered a complex conflict sustained by ties of loyalty, family and retribution.

A car-bomb attack in a quiet suburban street brings back unwelcome memories for Jack, when evidence points to the involvement of a Northern Irish terror group, long thought inactive. As the family of the victim seek their own form of justice, Jack calls an old friend to help them find those responsible and stop the act of revenge from escalating.

The team struggle to find the final piece of the puzzle left behind by the murders in Brighton. At home, Nikki and Matt discuss their future together.

A collection of dismembered body parts, covered in intricate Japanese tattoos, have been found in a Brighton waste dump. The team are called in to painstakingly piece them together and they search for clues to lead them to the killer. Outside of the lab, Nikki has some news she desperately wants to share with Matt.

When tragedy strikes at the heart of the Taylor family, it puts Noah on a collision course with the criminals behind the drugs. Meanwhile, one of the team's life is endangered when they become the victim of a knife attack.

After a teenage boy dies in a meth lab explosion, his inspirational head-teacher, Noah Taylor, resolves to confront the drug dealers preying on his community. Dangerous batches of new drugs are hitting the streets and its down to Nikki, Jack, Clarissa and Thomas to deal with the consequent rise in overdoses. When tragedy strikes at the heart of the Taylor family, it puts Noah on a collision course with the criminals behind the drugs.

A slip-up from DCI Cramer results in the premature release of a suspect - and when his mistake becomes evident in the light of incriminating evidence, he tries to blame the Lyell for the oversight. With the team under increasing scrutiny, Thomas offers to bear the brunt of the pressure while the team focus on the search for the perpetrator.

When his career in the field is threatened, will Thomas leave the Lyell to save his team?

Fresh from her trip to the States, Nikki and the Lyell unit are called in to investigate an attack against a trans man. When two further attacks are reported, one leaving a potential witness, it becomes clear that the team is facing an outbreak of violence against the trans community.

With the Lyell under increasing financial scrutiny and frustration growing between the team and the acting DCI, can they pull it together to find the killer before they strike again?