The TARDIS has barely landed in an alien sewer when a distant scream sends Susan racing to give aid, and the crew split up.

Trying to reunite, the travellers find themselves in something resembling a monastery led by a man half-way between an Abbot and a warlord. They discover that they are in Urth, a barbaric place clinging on to its former glory.

It's somewhere its populace are never allowed to leave, somewhere keeping many secrets from its people.

And today those secrets will be revealed...