In 1983 Dr Alec Jeffreys, a lecturer at Leicester University, enthuses to his students and less than impressed wife Sue, on the wonders of DNA. At the same time Chief Superintendent David Baker investigates the murder of school-girl Lynda Mann, setting up his base in the area where she was killed, but failing to make any progress, so that DCC Chapman orders him to concentrate on other crimes. Two years later a newspaper article highlights Jeffreys' discovery that every individual has their own unique DNA and he is called upon to help in a deportation case , which, due to his efforts, is successful. But in 1986, with Lynda Mann's murder a cold case, another young girl, Dawn Ashworth, is killed in the same way and in the same locality. Teen-ager Gavin Hopkirk is the prime suspect and admits to killing Dawn but not Lynda. Hearing of Jeffreys' success Baker seeks him out to prove that Hopkirk killed both girls but Jeffreys' findings reveal that Hopkins did not murder either of them. Written by don @ minifie-1

As Baker explains to the press why Hopkirk was released Jeffreys becomes a reluctant celebrity - though not popular with the police, who believed they had their killer. The trail goes cold until Baker has an idea, based on a previous case and proposes to Jeffreys the mass testing of 5,000 local men between 15 and 34 to match their DNA with that found on the victims. Chapman is sceptical, suggesting objections and civil servant Joy Blakefield questions the cost, half a million, though support comes from an unlikely source, prime minister Thatcher. The testing goes ahead though there is resistance in the community until the bereaved Mrs Ashworth speaks out. Ultimately the killer is trapped when, in accordance with one of Chapman's objections, he attempts to persuade a friend to take his place at the testing and the deception is exposed - heralding a new age in police detection. Written by don @ minifie-1