A blackmail plot at the end-of-season wrap party for middling detective series Bankwood turns fatal - because another member of the cast has a plan of their own. The setting: a spectacular underground Masonic Hall beneath the City of London, with many tunnels, no phone signal ... and only one way out. Unlike the programme, there are no killers or psychopaths, but a disparate group of people who all have a secret they'd rather stay hidden - and will go to extreme lengths to keep under wraps.