Nikki is arrested when a former colleague she came to blows with is murdered. Tony fears that his son David may be involved in the killings, but Nikki has her suspicions about someone once close to her.
When a young British Muslim returns to the UK in secret it is thought she may be connected with drug smuggling, but when a leading anti-fundamentalist Muslim is murdered something more sinister is feared.
When an ex-convict is found murdered in his home suspicion falls on a recently released child killer. Both men are clients of forensic psychologist Sasha Blackburn, whose work with the men is called into question.
As further victims are murdered the Lyle team revisit a 15-year-old case that leads them to believe that Paul is not who he says he is. The team turn to Sasha for help but has her viewpoint been compromised?
When the body of an armed police officer is found, the team join the search for the killer. Jack discovers a link to London's gun trade and puts himself in danger to discover the truth about the death of the young Pole.