S1, E1: First of a two-part drama telling the true story of how a brilliant scientist and pioneering detective united to create the most significant advance in modern criminal investigation. In 1984, Dr Alec Jeffreys, a University of Leicester scientist, unlocks a remarkable technique to read every individual's unique DNA fingerprint. Four miles away, DCS David Baker is a tenacious detective leading the search for the murderer of a schoolgirl. Two years later, a second schoolgirl is murdered and a local teenager is arrested. He confesses to the second murder, but denies the first. Certain that the same man murdered both girls, DCS Baker approaches Dr Jeffreys, whose DNA test has only been used in immigration and paternity cases. Can the test prove the teenager murdered both girls? The community wait anxiously for news that the killer has been caught.