The discovery of Oliver Houghton in a London sewer has UCOS re-examining the murder of artist David Straka 20 years ago. Houghton was a potential witness in Straka's still unsolved murder and Sasha's ex-husband, DAC Ned Hancock, doesn't have the resources to follow up any possible connection between the two deaths. Straka was found dead on Hampstead Heath and drowned after his face had been pushed into a small stream. Steve sees a slight connection - the stream Straka was killed in ultimately flows into the sewer where Houghton was found. A collection of film clips Straka had edited together and Steve's knowledge of local history provide the clues that lead them to an occult group who, 40 years ago, accidentally killed a young woman during one of their ceremonies and which eventually leads to the murders of Straka and Houghton.
—garykmcd