When the investigation into a Belfast architect's murder goes cold, Detective Superintendent Stella Gibson arrives from London to lend her expertise. At the same time, Paul Spector outwardly a grief counselor and family man, secretly a depraved killer plans his next attack, on young lawyer Sarah Kay.

Stella refuses to speak to local reporter of her involvement in the Monroe case. She is then called to the murder scene of Sarah and asks to take over the investigation. She learns from the pathologist that Sarah was pregnant. Meanwhile, the murder of drug-dealer Michael Lockwood seems to spark off a retaliatory murder.

Stella holds a meeting to alert her officers that they are dealing with a serial killer. She gives a press conference before selecting her team and giving an accurate profile of the murderer. Spector goes with his family to visit his in-laws but makes a point of studying Stella on her television appeal. Soon afterwards Spector selects his next victim.

Spector sends an anonymous letter to Sarah Kay's father, claiming he would not have killed her if he had known she was pregnant. Stella allows Mr Kay to make a public appeal. A woman comes forward and tells Stella she once met a one night stand who tried to strangle her whilst having sex and wonders if he is the murderer. Meanwhile, Spector murders his next victim and her partner.

Spector's latest victim, Annie Brawley, survives. She is taken to hospital and tested for DNA on her body. Spector's wife sees him on CCTV footage of Sarah Kay's last recorded movements and makes him go to the police to eliminate himself from enquiries. Stella receives a bizarre phone call from Spector, promising to end his killing spree, though that is not the end of the matter.