Nikki and Jack investigate a man run down and killed by a tube train. When a female vicar is then murdered after being followed on the underground, the team face the difficult prospect of building a case against her distraught husband. Nikki forms a bond with high-flying DI Luke Nelson, impressed by his intelligence and compassion. He asks her to help re-evaluate the details of his father's murder when he was a child. When a young roofer is stalked and brutally killed on his journey home, Nelson and the Lyell team identify a suspect on the tube cameras who links the murders, and a full-scale manhunt is launched.