The King's Demons is the sixth and final serial of the 20th season of the British science fiction television series Doctor Who, which was originally broadcast on BBC1 on 15 and 16 March 1983.
In the serial, the alien time traveller the Master (Anthony Ainley) uses the robot Kamelion to imitate King John of England as part of the Master's plot to have the real King John overthrown and thus prevent the signing of Magna Carta.
To defend his honour his son Hugh takes on the King's champion, Sir Gilles Estram, in a joust.
It seems the King's champion is not who he claims to be, either: Sir Gilles sheds his disguise and reveals himself to be the Doctor's nemesis, the Master.
The monarch is really Kamelion, a war weapon found by the Master on Xeriphas, which can be mentally controlled and used to adopt disguises and personas.
Nathan-Turner was so impressed he commissioned scriptwriter Terence Dudley to develop a storyline to introduce Kamelion into the series.
To hide Ainley's return, Sir Gilles was credited in the Radio Times as having been played by "James Stoker", an anagram of "Master's Joke".
The King's Demons was released on VHS in November 1995 in a box set along with a special edition of the subsequent serial "The Five Doctors" and a postcard book.