Doctor Who - The King’s Demons, by Terence Dudle, Target, 1986.
Number 108 in the Doctor Who Library. 153 pages, paperback. Original script by Terence Dudley, BBC, 1983.

This adventure features the 5th Doctor, Tegan, and Turlough.

SUMMARY
March 4, 1215:  The TARDIS materializes in the middle of a jousting match between King John’s champion, Sir Giles Estram, and Hugh Fitzwilliam, son of Ranulf Fitzwilliam, who has retired to his castle after years of service to the King in the Holy Land. King John has become a cruel, spiteful, boastful tyrant and has taken over Ranulf’s castle. The TARDIS’s arrival startles everyone but the King, who welcomes the trio of travelers as his “demons.”
Over the course of two days, loyalties shift back and forth as Sir Giles reveals himself as the Master, who has disguised his TARDIS as an iron maiden. It seems he escaped entrapment on Xeriphas (see “Time Flight,” by Peter Grimwade) with the help of an AI android that can change its appearance. It is appropriately named Kamelion and is controlled somewhat by telepathy by a user.
The Master’s plan is to alienate the King’s barons against the King to prevent signing of the Great Contract, which will become the Magna Carta, thereby halting the development of democracy. The Doctor outwits the Master at the last minute and leaves with Kamelion in his TARDIS.

OPINION
Terence Dudley writes a masterful (sorry, I had to) novel from his own script. I barely felt the 153 pages that came from the 2 part televised story. It’s not a strong story and begs the question of the Master’s motive. Why would the Master care about democracy in Medieval Europe? Unless it was just for fun, it really doesn’t make any sense. Still, the book is very well written and Dudley makes good command of language in the narrative.