Captain Revere is the latest victim to be sucked beneath the surface of Frontios, leaving the young Plantagenet to run the colony. The TARDIS has also been 'destroyed'...

Trying to discover why Frontios is being attacked from both above and below, the Doctor faces the malevolent Tractators while revolution brews in the colony...

Turlough's race memories of the Frontios threaten to overwhelm him as the Doctor finally convinces Brazen that the threat from underground is very real...

The Doctor manipulates the Gravis into restoring the TARDIS. But he must find a way to break the Tractators' hold over Frontios before they wipe out all life on the surface.