Monk, himself a former marathon runner, finds himself involved in a murder case in which one of the suspects is a running legend.
The Master arrives, posing as an adjudicator from Earth who has come to settle the dispute between the colonists and the miners. However, he has an agenda of his own. Meanwhile, the Doctor discovers a doomsday machine.
The Interplanetary Mining Corporation, represented by Captain Dent, arrives on the planet claiming that the colonists have no right to be there. The Doctor discovers too much about the mining schemes of Captain Dent and is marked for death.
With Jo being held as a 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.
A godlike being (Guy Rolfe) offers the Alphans a chance to live on a planet resembling Eden.