In 1984, 58-year-old Mary McLaughlin was found dead in her home in Glasgow. She had been strangled with the tie of her dressing gown. Mary was last seen walking home from the pub at night, with an unknown man following her. Despite the best efforts of the investigating officers, they couldn’t identify the unknown man and the trail would run cold for decades. In 2014, advances in DNA profiling gives new hope to cold cases. Police Scotland detectives and forensic scientists reinvestigate the case, sampling items preserved from the original crime scene to try and find Mary’s killer. A DNA hit leads to a potential suspect, but even more questions arise when this person appears to have an ironclad alibi. The case won’t be solved by forensic science alone, and detectives need to go back to basics and hunt in the archives for clues to figure out who killed Mary.

In the Highland town of Elgin, detectives face a challenging investigation as they work tirelessly to uncover the truth behind Kiesha Donaghy’s brutal murder.