The Sixth Doctor and Peri visit the planet Varos to obtain supplies of a rare ore called Zeiton 7, vital to the functioning of the TARDIS. Varos was once a colony for the criminally insane and the descendants of the original guards still rule, while the poverty-stricken people are kept entertained by screenings of public torture from the Punishment Dome.
Their Governor has been trying to negotiate a better export price for zeiton ore from Sil, an envoy of the Galatron Mining Corporation, whose reptilian body is supported and kept cool by a mobile water tank.
The Doctor and Peri meet two rebels, Jondar and Areta. Peri and Areta are captured and almost reshaped into beast-like creatures by Quillam, the Dome's sadistic commandant, but the Doctor saves them and tells the Governor the true value of zeiton 7.
Quillam and Varos's Chief Officer, who are in the pay of the Corporation, try to kill the Doctor and the Governor but are themselves despatched. Sil plans an invasion of Varos by a force from his home world, Thoros-Beta, but the Corporation veto this and instruct him to buy the zeiton ore at any price.