The Doctor attempts to put a halt to the drilling project.
The Doctor must return to his world to warn it of the danger. But time is running out.
The Earth’s crust has been penetrated, releasing terrible forces from within.
Arrested as a spy the Doctor is unable to convince anyone of the danger facing them.
The Doctor is transported to a parallel world where drilling is at a more advanced stage.
The green slime oozing from the drill head is transforming men into monsters.
The Doctor and UNIT are at a drilling project intending to penetrate Earth’s crust.