Synopsis
The story revolves around the lost planet Shada, on which the Time Lords built a prison for defeated would-be conquerors of the universe. Skagra, an up-and-coming would-be conqueror of the universe, needs the assistance of one of the prison's inmates, but finds that nobody knows where Shada is any more except one aged Time Lord who has retired to Earth, where he is masquerading as a professor at St. Cedd's College, Cambridge. Luckily for the fate of the universe, Skagra's attempt to force the information out of Professor Chronotis coincides with a visit by the professor's old friend, the Doctor.
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