Three Time Lords meet at an observatory and discuss the theft of confidential files relating to "the Doomsday Weapon." They begrudgingly realise that only one man can help them — and the Doctor, accompanied by Jo, is temporarily released from his exile and sent in the TARDIS to the desert planet of Uxarieus in the year 2472. There he finds an outpost of human colonists living as farmers. The colony is not a success — the land seems unusually poor and recently they are being besieged by representatives of rapacious mining corporations, and more recently, ferocious reptiles. The colony's governor, Robert Ashe, makes them welcome, and explains the colonists fled a year ago to the planet to escape the overcrowding and pollution on Earth.
Two colonists die in a reptile attack that night, and the next morning a man named Norton arrives at the settlement, claiming that he is from another colony that was wiped out by the reptiles. While the Doctor is investigating the dome of the dead colonists he is surprised by a mining robot controlled by Caldwell, a mineralogist for the IMC. Caldwell invites the Doctor to talk to his bosses and hear their side of the story. His superior, Dent, is a ruthless mining engineer, who has been using the mining robot to scare and now kill the colonists - something which Caldwell finds repellent. Dent knows the planet is rich in rare minerals and wants it for IMC and his greedy troops agree that this should be done at any cost.