Serial: 6J  Episode Nos. 606 and 607.  Title: The King's Demons

The TARDIS lands in the middle of a joust attended by King John, who to the Doctor's surprise welcomes the travellers as his "demons."  Invited to attend him, the Doctor becomes suspicious that this is not the real King John, suspicions which are further fuelled when he learns the King's Champion is the Master in disguise...

This story sees the debut of Kamelion, voiced by Gerald Flood.

starring Peter Davison as the Doctor, Janet Fielding as Tegan, Mark Strickson as Turlough, Anthony Ainley as the Master, and Gerald Flood as the Voice of Kamelion. Written by Terence Dudley, Produced by John Nathan-Turner, and Directed by Tony Virgo.

Originally transmitted from 15 March 1983 to 16 March 1983 on BBC1.
 

Serial: 6K  Episode No. 608.  Title: The Five Doctors

A mysterious Player kidnaps the Doctor and his previous incarnations and deposits them in the Death Zone on Gallifrey, with deadly traps and old enemies standing between them and the fortress Dark Tower at the center.  Can they survive to reach the Tomb of Rassilon itself and confront the Time Lord who brought them there?

This is the special 20th anniversary story.

starring Peter Davison, Jon Pertwee, Patrick Troughton, Richard Hurndall, Tom Baker, and William Hartnell as the Doctor, Janet Fielding as Tegan, Mark Strickson as Turlough, Elisabeth Sladen as Sarah Jane Smith, Nicholas Courtney as Brigadier Lethbridge-Stewart, Carole Ann Ford as Susan Foreman, Lalla Ward as Romana, and Anthony Ainley as the Master, with cameos by Richard Franklin as Captain Yates, Caroline John as Liz Shaw, Wendy Padbury as Zoe, Frazer Hines as Jamie, and John Leeson as the Voice of K9. Written by Terrance Dicks, Produced by John Nathan-Turner, and Directed by Peter Moffatt.

Originally transmitted on 23 November 1983 on PBS and 25 November 1983 on BBC1.