It is discovered that the Master has stolen secret Time Lord files on the Doomsday Weapon.  The Time Lords send the Doctor to return the files to them.  The Doctor and Jo are sent to the planet Uxarieus in 2472 where they find themselves in the middle of a dispute between colonists and miners from the IMC.  The Master arrives as an Adjudicator to decide the the dispute between the miners and colonists. 

The Master and the Doctor go to an underground city and find a machine that turn out to be the Doomsday Weapon.  The Doctor persuades the Guardian of the machine to destroy it and the Doctor and Master escape.  The Master then uses his TARDIS to leave the planet while colonists overcome the IMC crew make them surrender.