Aired April 10 - May 15, 1971
Written by Malcolm Hulke
Directed by Michael E. Briant
Upon discovering that the Master has stolen a secret file regarding a Doomsday Weapon, the Time Lords decide the use the Doctor as an unwitting agent. They will allow him to travel in time and space for the first time since he was exiled to Earth and use him to stop the Master's scheme. The TARDIS takes the Doctor and Jo to the planet Uxarieus in the 25th century. They encounter a group of colonists who are barely surviving on the planet. Their crops refuse to grow, they have a shaky relationship with the primitive indigenous people, and they are worried that the Interplanetary Mining Corporation may try to chase them off.
Sure enough, the Doctor soon uncovers a plot by the IMC to do just that, as the company has discovered a plentiful deposit of duralinium exists on the planet. The mineral will provide much needed building material to a dangerously over-populated Earth. The IMC ship's crew will do anything, including murder, to force the colonists off-planet. However, the colonists don't have the resources to survive moving on, if they leave, they'll likely die.
As he tries to resolve this conflict, the Doctor also learns that the indigenous Primitives and their High Priests worship a large machine tended by a creature called the Guardian. The primitives were not always so, they apparently at one point had a technologically advanced society.
The colonists are pinning their hopes on the arrival of an Adjudicator from Earth to resolve their conflict with the IMC. When the Adjudicator arrives, however, the Doctor is shocked to discover it is in fact the Master. After hearing both sides, the Master rules in favour of the IMC, who promptly take over and begin the process of forcing the colonists off-world.