In 1983, in a small village outside Leicester, 15-year-old Lynda Mann is found by a footpath, raped and strangled to death. A year on, after an exhaustive but fruitless search for the killer, Detective Chief Superintendent David Baker is forced to scale down the investigation. Meanwhile, just a few miles up the road at the University of Leicester, scientist Dr Alec Jeffreys invents a remarkable technique to read DNA – the unique genetic fingerprint of every individual – something never previously achieved despite decades of research across the globe. Summer 1986, and 15-year-old Dawn Ashworth goes missing – last seen just a hundred yards from where Lynda's body was discovered. Dawn's body is found two days later, she has been strangled and hidden in undergrowth near a footpath shortcut. Reading about Jeffreys' work in a local paper, Baker approaches him at the university – perhaps the DNA test can prove the teenagers involvement in Lynda's death?