The Time Lords discover that the Master has stolen their secret file on the Doomsday Weapon and decide to send the Doctor to retrieve it for them. The TARDIS takes the Doctor and Jo to the desolate planet Uxarieus in the year 2472. There they become involved in a dispute between some beleaguered colonists and the crew of an Interplanetary Mining Corporation spaceship over the ownership rights to the planet. The Doctor learns that the indigenous Primitives and their High Priests worship a large machine tended by a creature called the Guardian.