Tension on the tube network threatens to boil over, and security is stepped up. The accidental verdict in the first death is called into question and the witnesses are brought in for interview. Amongst them is Lana Sutherland, a young hotel worker who has taken in a homeless man, Owen. DI Nelson lets Lana slip through the net before he realises she may be harbouring a dangerous killer. Nikki helps Nelson unearth the mystery of his father's death, allowing him to confront and resolve the childhood memories that haunt him. He refocuses on the case and, with Jack and Nikki, discovers the missing key that links the victims.