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.
Written By: Jonathan Morris
Directed By: Nicholas Briggs
Tom Baker (The Doctor), Lalla Ward (Romana), Matthew Cottle (Mr Maxwell Wilberforce Bell), Abigail McKern (Mrs Lucretia Bell), Robbie Stevens (Mr Jelicho Wigg/ Mr Wilfer Wagstaff), Jane Slavin (Miss Arabella Wagstaff/ Mrs Betsy Wagstaff), John Banks (Silex/ Mr Stoker). Other parts played by members of the cast.
Producer David Richardson
Script Editor John Dorney
Executive Producers Jason Haigh-Ellery and Nicholas Briggs