The case of Gareth Williams, a GCHQ employee on secondment to MI6, who was found dead inside a padlocked duffel bag in the bath of his London flat in 2010. The case has become one of the biggest unsolved mysteries in recent British history, and even now, 12 years after Williams' death, the public are still waiting for answers as to what happened to the 31-year-old codebreaker. Through interviews with former detectives, forensic investigators, MPs and people who knew Gareth, along with archive footage and reconstruction, this documentary re-examines the case in a bid to get to the bottom of the events that lead to Williams' untimely death - asking did he act alone or were other forces involved?