DS Sarah Collins is called to the brutal stabbing of a teenager, but her investigation soon finds itself at cross purposes with a covert operation led by Kieran Shaw, who is working with Sarah's old partner Steve Bradshaw, now deep undercover. Meanwhile, Lizzie Adama is back at Farlow trying to balance her new role as a detective with family life, but her path and Kieran's are about to cross.

Kieran and Baillie share some information that could help Sarah make an arrest but her reluctance to cooperate leaves them exasperated. Shakiel puts Steve's loyalty to the test.

Operation Perseus reaches its climax as Kieran prepares to catch Shakiel with a car-load of automatic weapons. However, Spencer's killer, Jermaine King, has escaped from custody and kidnapped Ryan's sister, Tia. Conclusion of the drama starring Gemma Whelan and Jimmy Akingbola.