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!