Examining one of the most baffling unexplained deaths of recent years. In August 2010, Gareth Williams, a GCHQ employee on a 3-year secondment to MI6, was found dead inside a padlocked duffle bag in the bath of his London flat.

Susanna Reid explores a cold case enquiry into one of Britain’s most prolific serial killers. The film reveals the extraordinary game of cat and and mouse that Pembrokeshire detectives waged against the man they suspected had brutally murdered at least four people. He had eluded them for 20 years, but how would new forensics methods, and a fresh team of local detectives, now crack a case that had twice run cold?