4/7 Arrested as a spy the Doctor is unable to convince anyone of the danger facing them.
3/7 The Doctor is transported to a parallel world where drilling is at a more advanced stage.
2/7 The green slime oozing from the drill head is transforming men into monsters.
1/7 The Doctor and UNIT are at a drilling project intending to penetrate Earth's crust.
7/7 Carrington intends to turn the world against the aliens by revealing them on TV.