When his girlfriend wants to keep the light on while they have sex, Chris Hughes (Robert Glenister
) is upset. We soon see him walk out the door while she asks him to talk to her. He runs through the neighbourhood. Meanwhile, Susan Covington (Beatie Edney
) consoles her child. She goes downstairs and falls asleep in a chair. She is awoken by a sudden sound.
In the morning, Jane Tennison (Helen Mirren
) lies awake in her bed when the alarm sounds. Soon she leaves home in a taxi.
The same morning, a nanny who works for Susan Covington goes into Susan's house and finds her on the floor with blood streaming from her head. She goes upstairs and finds Susan's child missing from her crib. She calls emergency. The mother is soon taken to hospital in an ambulance and the baby, Vicki, is filed missing.
Jane Tennison is also in hospital. We learn that she has had a termination. She is eager to get back to work. She soon arrives at a police station to take in her office as superintendent. One of the files on her desk is that of the missing baby girl.
We briefly see Chris Hughes cutting out an article about the presumed kidnapping from a newspaper.
Tennison meets with investigators Richard Haskons (Richard Hawley
) and Tony Muddyman (Jack Ellis
). Her predecessor authorized the release of information to the newspaper the night before, and she is upset that it could hurt the investigation more than it helps. Muddyman says Susan Covington is conscious but suffers from amnesia. Tennison asks Haskons to arrange a media appearance. Tennison then addresses the staff and lets Haskons brief them on the case. The father of the child, John Warwick, left Susan months before; he doesn't have an alibi, but no trace of the child has been found at his farm. Forensic evidence is sparse. The team speculate on motive; since there was not much money to get at they rule out kidnapping and conclude that the child was abducted for nurture or abuse. Tennison has them look for known paedophiles in the area as well as people who have recently lost a child of their own.
Tennison talks to Vicky's nanny Carolyn (Tracy Keating
) about her routines and also takes a camera that Susan had bought for Carolyn. Meanwhile, Susan's car, which was apparently stolen on the morning in question, is found empty close to Susan's house. Tennison goes to see Susan in hospital, and she gives her account of the night: When she woke up in her chair she went into the hall where someone grabbed her and knocked her into the wall. She also tells Tennison about a man accompanied by two girls who she met in the park and who was very keen on Vicky.
We see another brief clip of Chris Hughes, now in a swimming pool with two girls, his girlfriend's daughters.
Tennison has Susan appear on television to ask the abductor for her daughter back. Chris Hughes and his girlfriend watch. At the police station, it turns out Susan's car was caught speeding in the middle of the night going out of London, and the squad try to pin down a search radius. Tennison phones Chief Superintendent Kernan (John Benfield
) to get more officers assigned to the case, but he refuses. At night she goes to the crime scene to look around. When she answers the phone we see the man on the other end, but he is silent.
The next day Tennison and Haskons go north to see Susan's ex, Warwick (Adrian Lukis
); we see that it was him on the phone the night before. He says he hasn't been to London since he left Susan, but Tennison shows him a petrol receipt that shows he was there the past weekend. He explains that he was with a woman he is having an affair with. Back at the station, the squad has a photo from Carolyn's camera which may include the man Susan talked about. They also find Chris Hughes in a database of paedophile convicts and discover that he lives near Susan. They go search his bedsit and bring him in for questioning. Tennison conducts the interrogation and asks him about some of their findings: a photo album with pictures of young children, though all of them dressed and proper, and a scrapbook with newspaper clippings from child murder and kidnapping cases. He denies any involvement in Vicki's disappearance. Susan, however, confirms that he is the man from the park.
Muddyman and another officer questions Hughes' girlfriend, Anne Sutherland (Lesley Sharp
), who gives Hughes an alibi for the night excepting a short time when he left her house after they had a fight. The police must still find evidence to link him to the abduction. Tennison and Muddyman drive to Bedford to visit a clinic where Hughes stayed after he was convicted in a child assault case. They speak to the main psychiatrist, Dr. Schofield (Stuart Wilson
), who tells them he doubts Hughes is the abductor, specifically because Hughes didn't go for children that young. Susan has been escorted to her home for the first time since the abduction, and Tennison goes to see her; Susan begs her to bring back Vicki.
The baby girl's body is found in a lake, naked and lifeless. Tennison has the mother identify Vicki. Chris Hughes is released after 24 hours have passed since his arrest. Dr. Schofield comes to see Tennison, warning her that the pressure put on Hughes might drive him to new crimes. He shows her a videotape of Hughes talking about and justifying molesting a girl in the case he was previously convicted for. We see Hughes returning home to Anne; he lies to her about why he was arrested.
Tennison listens to the results of the autopsy; Vicki was suffocated. Muddyman takes Hughes's file from the station and brings it round for Anne to see while Hughes isn't there. Anne watches the videotape of her boyfriend talking about abusing children. Soon, she is at the police station changing her statement; Hughes no longer has an alibi for the night in question. The police go to the garage Hughes works at to pick him up again, and when he runs, Muddyman chases him down and knocks him about. Hughes falls and is run over by a car and has to be taken to hospital. When the ambulance is involved in an accident Hughes escapes. He soon turns up at Anne's house, which is unguarded. When Tennison learns that Hughes has escaped she sends Haskons, but too late: Hughes is barricading the house and holding Anne and the girls hostage. The police set up a siege; they get Hughes on the phone, but with no result. At the station, Muddyman is anxious about the situation. Tennison reprimands him for his behaviour at the arrest and also learns that he revealed confidential information to Anne to persuade her to testify against Hughes; she thinks it will make it impossible to get a conviction. When Muddyman breaks down and cries, Tennison guesses that he too has once been the victim of sexual abuse. He repeats the words, "I'm disgusting."
At the house, Hughes tries to convince Anne that he is not like he was before, that he loves her and wouldn't hurt the children. He speaks to the police on the phone and demands to talk to Tennison. She arrives and speaks to him on the phone. He claims he had nothing to do with Vicki's death and has Anne say that he was in fact with her on the night of the abduction. Tennison convinces him to let the hostages go and promises him that the police snipers will be withdrawn and that he will not be hurt.
Meanwhile, Susan Covington has left her house and arrives at the siege; she says she wants to speak to Hughes. Hughes slowly comes out the door with Anne and the girls around him. Tennison faces him and tells him that he can trust her, but when he sees snipers still in place he runs back into the house with one of the girls. A shot is fired, but no one is hurt. Tennison complains to the officers in charge of the siege that her wishes were not followed. Then she speaks to Susan. Tennison tells her she thinks Hughes is innocent of taking Vicki. Susan mentions how she caused the situation by identifying Hughes and how she doesn't want the girls to get hurt. Then she starts talking about how she only ever had a baby because it was what society expected of her and other women. She says that her own life suffered after she had Vicki, that she is the real victim. She talks about the night Vicki disappeared, and we realize that she is confessing that it was she who killed her daughter and dumped the body. She is distressed and starts banging herself on the head while saying she's sorry; Tennison holds her. The hostage situation is resolved, and Chris Hughes is taken away in an ambulance.
Jane Tennison and Tony Muddyman are seen appearing before a disciplinary committee. Jane returns home and sits on the couch staring into the air.