Series 20: The King's Demons

In 1215, the Court of King John of England is at the castle of Sir Ranulf Fitzwilliam to extort more taxes, and when the lord refuses to pay the King insults him. To defend his honour his son Hugh takes on the King’s champion, Sir Gilles Estram, in a joust. The latter wins easily, though the joust is disturbed by the arrival of the TARDIS. The Doctor, Tegan, and Turlough are greeted as demons and welcomed by the King.

Having established the date, the Doctor concludes the King is not himself - in fact, he is not the King at all, as he is actually in London taking the Crusader’s Oath. Sir Geoffrey de Lacy, the cousin of Sir Ranulf, arrives at the castle and confirms he knows the King is in London. Sir Gilles is about to torture him as a liar during a royal banquet when the Doctor intervenes. 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 arch nemesis, the Master. He flees in his own TARDIS, which had been disguised as an iron maiden.

The King knights the Doctor as his new champion, and he is given run of the castle. After a series of mishaps, including the death of Sir Geoffrey at the Master’s hands, the Doctor confronts the King and the Master and discovers the truth. 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. Disguised as King John, the Master intends that Kamelion will behave so appallingly so as to provoke a rebellion and topple the real King from his throne, thus robbing the world of Magna Carta, the foundation of parliamentary democracy. It is a small plan on the Master’s usual scale, but nevertheless particularly poisonous to the normal progress of Earth society.

The Doctor resolves the situation by testing the Master in a battle of wills for control over Kamelion. He takes control of the robot and steals it away in the TARDIS, thus foiling the Master’s scheme. Kamelion reverts to its robot form and thanks the Doctor for his assistance and rescue.

Cast
Doctor Peter Davison (Fifth Doctor)
Companions Janet Fielding (Tegan Jovanka)
Mark Strickson (Vislor Turlough)
Gerald Flood (Kamelion)
Guest stars
Anthony Ainley — The Master / Sir Gilles Estram
Gerald Flood — King John
Frank Windsor — Ranulf
Isla Blair — Isabella
Christopher Villiers — Hugh
Michael J. Jackson — Sir Geoffrey de Lacey
Peter Burroughs — Jester

Production
Writer Terence Dudley
Director Tony Virgo
Script editor Eric Saward
Producer John Nathan-Turner
Executive producer(s) None
Production code 6J
Series Season 20
Length 2 episodes, 25 minutes each
Originally broadcast March 15–March 16, 1983

source: wikipedia