2/5 Devil's End is sealed off from the outside world by an invisible heat barrier.
1/5 The Doctor tries to stop Professor Horner from breaking into a barrow near Devil's End.
The Doctor and the Master confront the guardian of the Doomsday Weapon.
The Master forces the Doctor to take him to the underground city of the primitives.
The Master pretends to have come to settle the dispute between the colonists and IMC.