When tragedy strikes close to home, it puts head teacher Noah Taylor on a collision course with the criminals responsible. Can the Lyell team help him stop the cycle of violence?

With new drugs hitting the streets, a head teacher confronts the dealers preying on his community, as the team deal with a surge in drug-related crime.

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.

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.

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.

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.

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.

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.

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.

A mistake has been made during the course of the investigation, and Thomas is targeted as a scapegoat. As frustration grows between the Lyell and the acting DCI, can Thomas rally the team to gather crucial evidence to find the killer before they strike again?