It’s a troublesome place, difficult to administer, and as a piece of real estate it’s worthless because by definition there’d be no one to sell it to.
The Doctor discussing the Universe with Skagra, episode 6
An aged Time Lord, Professor Chronotis, has chosen Cambridge University as his retirement home, content to live the remaining years of his final incarnation as a humble college lecturer. But an ambitious alien scientist, Skagra, has recently learned that Chronotis knows the whereabouts of Shada, the Time Lords’ prison planet where a renegade of their race, Salyavin, has been imprisoned for centuries. Skagra has developed a flying sphere, which can steal the mind of anyone brought near it and transfer that person’s knowledge into himself.
The Doctor is ‘summoned’ to Cambridge by Professor Chronotis. He wishes the Doctor to return a book in his possession back to Gallifrey. This book is an artefact dating back to the foundations of Time Lord society, and contains forbidden knowledge, including the incarceration of Salyavin. His ‘crime’ was a talent for putting his mind into the minds of others, making them effectively him. Skagra intends stealing that ability and using it to create a single god-like version of himself through possessing the mind of every creature in the Universe.
But while the Doctor goes to retrieve the book, Skagra confronts Chronotis and uses his sphere to steal part of the Time Lord’s mind. However, Skagra finds he is unable to decipher the Professor’s thoughts without reference to the book and the mind of a lesser Time Lord: the Doctor’s perhaps.
A lengthy battle of wits concludes with Skagra reaching Shada ahead of the Doctor, Romana, Chronotis and K-9, and locating Salyavin’s cell. But Salyavin has long since escaped, regenerated and gone into hiding, atoning for the follies of his earlier self by helping others – disguised as a humble Cambridge professor …
Skagra uses the sphere to attack Chronotis again, acquiring his talent, and using it to bend the will of every other prisoner on Shada to his own. The Doctor counters by constructing a mind-projecting device of his own that stalls Skagra’s power long enough for Romana and K-9 to cause an explosion that wrecks Skagra’s concentration. Their minds freed, the prisoners turn on Skagra, forcing him to retreat aboard his ship. But the Doctor has reprogrammed the ship’s computer, directing it to keep Skagra a prisoner on board for the rest of his life.
Having control of the sphere means the Doctor can now free all of the stolen minds and return them to their owners.