The TARDIS is going underground. When the Doctor and Romana find themselves buried beneath the surface of an alien world, they're soon swallowed up by a giant burrowing machine. This is where the inhabitants of this planet live - in huge, constantly moving, Drill-towns, chewing up the fuel and resources of the planet in order to survive.
But something else lurks in the earth. Something that feeds on the Drill-towns. Something that is relentless and will not stop.
The Silex are hunting.