English detectives team up with a pioneering scientist to crack a case of sexual assault and serial murder. In 1983, a quiet country village is gripped with fear as authorities search for the killer of 15-year-old Lynda Mann. Clueless, they start again when Dawn Ashworth is killed three years later. They enlist the help of Dr. Alec Jeffreys, a molecular biologist who uses his breakthrough technique of genetic fingerprinting to rule out one suspect by comparing his DNA with that of semen found on the victims bodies. Police set up a DNA dragnet to trap and convict the real killer. This 1986 murder case is the first to use DNA as evidence in a criminal case.