John Simm and David Threlfall star in this two-part ITV crime drama. Set in Leicester, DCS David Baker (Threlfall) investigates the brutal murders of two local schoolgirls during the 1980s. But when his investigation falters, the detective approaches geneticist Alec Jeffreys (Simm) to help him solve the case using pioneering methods of DNA fingerprinting. Convinced the killer is local and having ruled out one of their chief suspects, the pair conduct the first ever search using DNA testing while the community becomes increasingly worried that a killer is still amongst them.