Explosive documentary series. Former police detective Mark Williams-Thomas discovers another brutal Glasgow murder that threatens to derail his investigation into serial killer Angus Sinclair. The Investigator previously linked Sinclair to the murders of Anna Kenny, Agnes Cooney and Hilda McAuley, who were all abducted and killed in similar circumstances within four months of each other in Glasgow in 1977. As the net closes in on Sinclair, Mark discovers another murder – that of 37-year-old baker Frances Barker – that bears all the hallmarks of the same killer. However, this murder is seemingly already solved; lorry driver Thomas Ross Young was arrested a week after Frances’s body was discovered and was later jailed for life for her murder. Mark delves into the murder of Frances to try and establish if Sinclair was involved. Is it possible that the police got the wrong man? If Mark can prove that Sinclair killed Frances, Sinclair would be one of Britain’s most prolific serial killers