Peter Davison (The Doctor), Janet Fielding (Tegan Jovanka), Mark Strickson (Vislor Turlough), Sarah Sutton (Nyssa), John Banks (Clifford Andrews), Alison Thea-Skot (Sally Lucas), Terry Molloy (Dr Wallace), David Seymour (Kevin), Andrew Dickens (Matthew/Major Harris), Charlie Norfolk (Caitlin Jones)
1983: as the country goes to the polls, two 'Urban Explorers', together with a freelance journalist, break into the long-defunct Cadogan Tunnels, once a secret wartime facility… and later, so rumour has it, the site of an experimental laboratory with a nasty sideline in vivisection.
What they find, in its twisting underground corridors, is something the most cynical conspiracy theorist could never have imagined: a highly-evolved society of questing, intelligent creatures, living right under humanity's nose for decades.
But there's no way out of the tunnels - as the Doctor, Nyssa, Tegan and Turlough are about to discover when the TARDIS brings them, too, into the complex. It's a rat trap - and they've all been caught!
(Duration: 120' approx)