The TARDIS takes the Doctor and Jo to a barren world in the far future where human colonists are scratching a living. But at night, the colonists are menaced by a monster.
The Interplanetary Mining Corporation, represented by Captain Dent, arrives on the planet claiming that the colonists have no right to be there.
When Jo attempts to break into the IMC ship to find incriminating information, she is captured and held hostage. If the Doctor doesn't co-operate, Captain Dent will have Jo killed.
The Master arrives, posing as an adjudicator from Earth who has come to settle the dispute between the colonists and the miners. However, the Master has an agenda of his own.
With Jo held prisoner in his TARDIS, the Master forces the Doctor to take him to the underground city of the primitives. Any sign of treachery from the Doctor and Jo will die.
In the underground city, the Doctor and the Master confront the guardian of the doomsday weapon. The miners make their last move to force the colonists to leave the planet.
