The King's Demons

From: http://www.bbc.co.uk/doctorwho/classic/episodeguide

The TARDIS materialises in 13th Century England during a joust held in the presence of King John. The King welcomes the Doctor, Tegan and Turlough as his 'demons', but his actions toward the family of his host Ranulf fitz William are decidedly hostile.

The Doctor discovers that what appears to be the King is in fact a shape-changing android called Kamelion whom the Master found on Xeriphas. The Master has disguised himself as the King's Champion, Sir Gilles Estram, and is using Kamelion in a plot to discredit the King and prevent the signing of the Magna Carta, thereby changing the course of history.

The Doctor manages to wrest control of Kamelion away from the Master, exposing his arch enemy's scheme.

5 out of 64 (7.8%) raters say this story requires a previous story.