In the early hours of 19 March 2011, 22-year-old Sian O'Callaghan left a nightclub in Swindon to make the half-mile journey back to her home. She was never seen alive again.The prime suspect in her disappearance, local taxi driver Christopher Halliwell, was arrested a few days later. Seemingly an ordinary family man, in reality, Halliwell regularly visited sex workers and viewed incredibly violent pornography. Chillingly, he had once asked a fellow inmate during a prison sentence in the 1980s how many people it was required to kill in order to be a serial killer.Not only did Halliwell admit to killing O'Callaghan, he then led police to the body of Becky Godden, a sex worker who had been missing for eight years. But as a result of an error in police procedure, this couldn't be used to charge him with Godden's murder. After a painstaking process of collecting new evidence, he was finally sentenced to a full-life jail term for the killings in 2016.However, essential questions remain: what happened in the eight years between Godden and O'Callaghan's deaths? Did Halliwell go quiet, or are there other innocent victims yet to be found? How far back could his killing spree go?This is the terrifying and gripping true story of one of the most brutal serial killers in modern times and the catastrophic error that almost prevented justice from being served.