The Doctor must find the most powerful book in the universe.
The Horns of Nimon was originally intended to be followed on transmission by another story - a concluding six-parter entitled Shada - but a recurrence of an industrial dispute that had previously caused difficulties during The Invasion of Time and The Armageddon Factor eventually led to its cancellation, even though extensive location filming in the Cambridge area and the first of its three planned studio sessions had already been completed.
Written by Douglas Adams as his final contribution to Doctor Who, Shada was envisaged as a Time Lord story without a Gallifreyan setting. It sees the Doctor bringing Romana to present-day Earth to visit Professor Chronotis, an elderly Time Lord who absconded from Gallifrey and now lives a quiet academic life at St Cedd's College in Cambridge. Also seeking Chronotis is a scientist called Skagra who has a device, in the form of a floating sphere, with which he intends to steal the Professor's mind and thereby learn the location of a book entitled The Worshipful and Ancient Law of Gallifrey. Skagra eventually succeeds in obtaining the book, which has been borrowed from the Professor's study by a student named Chris Parsons. He then kidnaps Romana and hijacks the TARDIS.
The book turns out to be the key to Shada, the ancient prison planet of the Time Lords. Skagra's objective is to use his sphere on one of the inmates, Salyavin, whose unique mental powers he can then exploit to project his own mind into every other creature in the universe. When the TARDIS reaches Shada, however, he discovers that Salyavin's cell is empty.
After a number of close encounters with the Krargs - Skagra's monstrous crystalline servants - the Doctor, Chronotis and K9, along with Parsons and his friend Clare Keightley, arrive on Shada in Chronotis' TARDIS, which has been disguised as his study. Chronotis admits that he is in fact Salyavin; he escaped from Shada centuries ago and has been living on Earth ever since. The Doctor thwarts Skagra's plans by winning a mind battle against him.
Shada was formally droppped from the seventeenth season in December 1979, it having become apparent that due to the backlog of Christmas specials waiting to be recorded there was no prospect of studio time being found for its completion. A number of attempts were subsequently made to remount it but these ultimately came to nothing, and in June 1980 it was officially cancelled. A version of the story was eventually released on video in 1992 with Tom Baker providing a narration to cover the missing scenes.
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