On 9 February 1983, Detective Chief Inspector Peter Jay discovers a drain clogged with rotting flesh and bones. The culprit, Dennis Nilsen, freely admits to the police that it's not just one or two bodies, but "15 or 16, I think".
With limited resources, Jay and the police continue to dig into Nilsen's memories in an effort to convict him for the murder of every one of his victims. But when Des withdraws his cooperation and stops talking, Jay fears justice will be lost.
Reeling from Des' change in plea, Jay and the police throw themselves into proving premeditation. If they can show the court that Des was undoubtedly of sound mind when he committed his crimes, he will go down and never be released.
Narrated by the star of new ITV drama Des, David Tennant, and timed to follow on from it, this documentary features insight from experts who attempted to get inside the mind of the murderer including criminologist David Wilson, who interviewed and corresponded with Nilsen over several decades.