Peter Davison continues his role as the Doctor, accompanied by Janet Fielding. While Nyssa leaves halfway through the season in Terminus, Mark Strickson arrives as new companion Vislor Turlough in Mawdryn Undead. In the penultimate serial of the season, the shape-shifting android Kamelion is invited aboard the TARDIS after the Doctor frees him from the Master. Kamelion accepts, though the character itself would only be seen again in Season 21's antepenultimate serial, Planet of Fire. Past Doctors return for the 20th Anniversary special, with Patrick Troughton and Jon Pertwee appearing as the Second Doctor and Third Doctor. The First Doctor returns, played by Richard Hurndall as original actor William Hartnell died in 1975. Tom Baker was asked to return to play the Fourth Doctor but declined. Past companions Elisabeth Sladen, Nicholas Courtney, Carole Ann Ford return for the 20th Anniversary special, whilst Nicholas Courtney also makes an appearance in Mawdryn Undead.
Anthony Ainley returns as the Master yet again in The King's Demons and The Five Doctors. The Black Guardian played by Valentine Dyall also makes a return in Mawdryn Undead, Terminus and Enlightenment. The Castellan played by Paul Jerricho appears in Arc of Infinity and reprises the role in The Five Doctors
Guest stars
Additional companions Frazer Hines, Wendy Padbury, Caroline John and Richard Franklin make cameos throughout the special. Returning villains for the season are Omega, Cybermen and The Mara. A lone Dalek and a lone Yeti appear briefly in The Five Doctors. David Banks makes his second of four appearances in the show in The Five Doctors as a Cyber-leader. Colin Baker, who was subsequently cast as the Sixth Doctor, made his first appearance in Doctor Who as Commander Maxil in the season's first serial, Arc of Infinity, becoming the first actor to appear in the programme prior to taking on the role of the Doctor.
Serials
To commemorate the twentieth season, the stories in this season involve the return of characters or villains seen in previous seasons. This season was broadcast twice weekly on Tuesday and Wednesday evenings on BBC1. It includes The Black Guardian Trilogy, consisting of the serials Mawdryn Undead, Terminus and Enlightenment and involving the arrival of Turlough, the departure of Nyssa and a single guest appearance from Brigadier Lethbridge-Stewart. Overall, these serials form a rough twelve-part epic. Although The Five Doctors was broadcast more than eight months after Part 2 of The King's Demons, which was the last regular story of the season, it is usually included as part of Season 20.
Broadcast
The entire season was broadcast from 3 January to 25 November. Transmission moved to Tuesdays and Wednesdays, except for the first episode of Arc of Infinity.
