The story begins in 1964. Married teenagers David and Maureen Smith have recently become parents to a baby girl called Angela. When their child dies of cot-death at the age of six months, Maureen turns to her older sister, Myra, for comfort, and David finds friendship in Myra's boyfriend, Ian Brady. David and Maureen know nothing about the secrets harboured by Brady and Hindley. Shortly after the tragedy, Hindley and Brady move with Hindley's grandmother Ellen Maybury, to a new council house; 16 Wardle Brook Avenue, on the Hattersley estate near Hyde. On the evening of 6 October 1965, David Smith witnesses a horrific murder at Brady and Hindley's house; the victim is a 17-year-old boy, Edward Evans. After the murder, David is forced to help clean up the mess and stay at the house until the early hours of the morning. When he returns home and tells Maureen about the crime, she finds his story hard to believe. In the morning, however, the couple go to the police, and Brady is arrested. Brady admits to murdering Edward Evans but insists that David Smith was a willing accomplice.
Police recover a suitcase full of incriminating evidence from a locker at Manchester Central Station, and quickly suspect that Edward Evans may not have been their only victim. Smith is soon taken in by police for questioning, as both Brady and Hindley have tried to shift the blame upon him, but the police soon determine that Smith did not take part in any murders. While questioning Brady and Hindley, the police read out the names of other missing children who have recently vanished in and around Manchester. To most of the names, Brady and Hindley respond "Never heard of him/her", with the exception of one: Pauline Reade, who had been Hindley's neighbour in Gorton. The evidence against the couple continues to mount; the most shocking comes in the form of pornographic photographs of a missing 10-year-old girl, Lesley Ann Downey, and a tape recording of the child's pleas for her life (these cries are not actually heard when the tape is played). The police find the bodies of both Lesley Ann Downey and John Kilbride buried in shallow graves on Saddleworth Moor. David Smith is questioned about the murders and Hindley and Brady try to convince the police that he was also involved, but he is soon released after no evidence is found to implicate him. On 21 April 1966, Brady and Hindley go on trial at Chester Crown Court, where they are greeted by a crowd of vigilantes. David Smith is the main prosecution witness, and Maureen, pregnant again, agrees to testify. During the trial, Lesley Ann Downey's mother, Ann West, and stepfather, Alan, barge into David and Maureen's apartment and attack them, believing that they were involved in Lesley Ann's murder. David and Maureen's unborn child is unharmed, while Mr. and Mrs. West are cautioned by the police. Brady and Hindley are convicted of the murders of Edward Evans and Lesley Ann Downey. Brady is also convicted of the murder of John Kilbride, and Hindley is found guilty for being an accessory to the murder. Brady is sentenced to three concurrent terms of life imprisonment, while Hindley receives two life sentences and a seven-year fixed sentence. The birth of their second child does nothing to alleviate public hostility towards David and Maureen. In fact, they find "Hindley Bitch" painted on their front door. Five years later, Maureen lives alone in an apartment while David has served a prison sentence for wounding a man who provoked him in a pub. David and Maureen have had two other children. Unable to cope with being a single parent, Maureen has placed her children into care, although David has now been released and has got them back. Having initially turned against Maureen for going to the police about Myra, Nellie tracks her down and the two reconcile. Nellie convinces Maureen to go visit Myra in Holloway Prison (Brady (who the trial judge felt had influenced Hindley into committing murder) has been imprisoned elsewhere). By now having rediscovered her faith in Roman Catholicism, Myra, whose bleached blonde hair has now turned brown, tells Maureen a day does not go by where she does not think about the suffering she helped bring on the children and their families, or the hell Maureen has been through because of her. Myra also explains that people are always making excuses for her behaviour, such as the beatings from her father when she was little damaging her, but Maureen says he used to beat her too, and she didn't do what Myra did. Myra also says some people say Ian Brady corrupted her, and that she had decided to sever all contact with him. David isn't pleased when he learns that Maureen has been to see Myra, but Maureen argues that Myra has changed. David and Maureen then turn their thoughts to their children and agree to patch up their differences for the children's sake. In the last scene, Maureen leaves the house. For a few moments, she stands on the doorstep, fighting back tears, and then she walks away from the house. Maureen is last seen walking down a street as the screen fades to black. An epilogue follows, revealing the fates of Maureen, David, Ian Brady, and Myra Hindley.