Shada

Shada: a prison built by the Time Lords for defeated would-be conquerors of the universe. A scientist named Skagra needs the help of one of the prison's inmates. He finds nobody knows where Shada is anymore, except one aged Time Lord who has retired to Earth, where he is a professor at St Cedd's College. Luckily for the universe, Skagra's attempt to force the information out of Professor Chronotis coincides with a visit by the professor's old friend, the Fourth Doctor.