On 16 October 1977, the bodies of two young women, Christine Eadie and Helen Scott, were found six miles apart in East Lothian. Both women were bound, sexually assaulted, and strangled with their own clothing. Only the night before, the two childhood friends had gone out for drinks in the World's End pub on Edinburgh's famous Royal Mile. Witnesses recalled Christine and Helen speaking with two unknown men that night. The hunt for the killers became one of the biggest manhunts Scotland has ever seen, as fear gripped Scotland and the press began to link the murders of Christine and Helen with unsolved killings in Glasgow.

In this episode, Ian Rankin explains that by the dawn of the 80s, the only hope of catching the World’s End killers lay with the people developing new techniques in forensic science. Astonishing archive footage reveals how policing underwent a revolution throughout the decade, fuelled by advances in forensic science, DNA fingerprinting and information technology. The programme also reveals how the FBI’s pioneering advances in forensic psychology led to hopes that psychological profiles of the World’s End killers could be created. Retired DCC Tom Wood reveals that he travelled to the FBI's headquarters with files on the unsolved murders from the late 70s in search of answers.