Rat Trap

1983: as the country goes to the polls, two 'Urban Explorers', together with a freelance journalist, break into the long-defunct Cadogan Tunnels. 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!