Fresh from her trip to the States, Nikki (Emilia Fox) 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. Thomas must rally the team to find the killer before they strike again.
After a teenage boy is killed 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 the effects are evident to Nikki, Jack, Clarissa, and Thomas as they deal with the rise in both violent and accidental deaths linked to the narcotics, while tragedy strikes at the heart of the Taylor family.
When a gang of dealers recruit young men to push spice and fentanyl near a London school, head teacher Noah Taylor has no choice but to confront them. DI Kate Ashton leads the police investigation, and as well as tracking down those responsible for the recent deaths, Ashton also seems keen to pursue the possibility of a relationship with Jack. When the pair search a suspect's flat, Jack gets caught up in a rising tide of violence, which culminates in several deaths. Nikki, Thomas, Jack and Clarissa must work to find those responsible before it's too late.
During a heatwave in Brighton, the Lyell team is summoned to a waste dump where a body part has been found. The search is on to recover the other dismembered remains and piece together the victim's body, which is covered in intricate Japanese tattoos. Nikki has some news that she desperately wants to share with Matt, but must wait until he visits from America.
With the remains of a second tattooed victim appearing in a Brighton recycling plant, it seems the Lyell team is on the trail of a serial killer. Working closely with local detectives DI Taramelli and DS Quicke, they must painstakingly piece together the evidence, and unearth the connections between a diverse range of suspects.
A shocking incident on a suburban street brings back painful memories for Jack, forcing him to revisit his past. As a member of the victim's family seeks justice of their own, forensic evidence discovered by the Lyell suggests the involvement of a Northern Irish terrorist group, long-thought inactive.
While the team searches for the killer, Jack works alongside an old friend who has dedicated her life to exposing the suspected organisation. 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. 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.