The twentieth season of British science fiction television series Doctor Who began on 3 January 1983 with the story 'Arc of Infinity', and ended 16 March 1983 with 'The King's Demons'. A 20th Anniversary special, 'The Five Doctors', followed in November 1983.

Peter Davison continues his role as the Doctor, as well as Janet Fielding (Tegan Jovanka). While Nyssa (Sarah Sutton) leaves half way 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 (voiced by Gerald Flood) was invited aboard the TARDIS after the Doctor freed him from the Master. Kamelion accepted, though the character itself would only be seen again in Season 21's ante-penultimate serial, 'Planet of Fire'.

Past Doctors return for the 20th Anniversary special, with Patrick Troughton and Jon Pertwee appearing as The Second Doctor and The Third Doctor. The First Doctor returns although is 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 (Sarah Jane Smith), Nicholas Courtney (Brigadier Lethbridge-Stewart), Carole Ann Ford (Susan Foreman) return for the 20th Anniversary special. Additional companions Frazer Hines (Jamie McCrimmon), Wendy Padbury (Zoe Heriot), Caroline John (Liz Shaw) and Richard Franklin (Mike Yates) make cameos throughout the special.

Anthony Ainley returns as the Master yet again in 'The King's Demons' and 'The Five Doctors'. Returning villains for the season are Omega ('Arc of Infinity'), The Black Guardian ('Mawdryn Undead', 'Terminus' and 'Enlightenment'), Cybermen ('The Five Doctors') and The Mara ('Snakedance'). A Dalek and a Yeti appear briefly in 'The Five Doctors'.

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. Within the season are the serials; 'Mawdryn Undead', 'Terminus' and 'Enlightenment' which are part of The Black Guardian Trilogy. These serials oversee the departure of Nyssa and the arrival of Turlough along with 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 he season, it is usually included as part of Season 20.

Total Episodes: 24 (including the 2 part 20th Anniversary Special)

01 - Arc of Infinity - 4 Episodes

     Synopsis: The Doctor is attacked by an anti-matter creature, which attempts
     to join with him genetically. Its intention is to bond with a Time Lord and
     cross over into this dimension through the Arc of Infinity — the gateway to
     all dimensions. The chosen Time Lord is the Doctor.

02 - Snakedance - 4 Episodes

     Synopsis: The arrival of the TARDIS on Manussa triggers nightmares in Tegan,
     who dreams of a snake-shaped cave mouth. It is evident to the Fifth Doctor
     that the Mara is reasserting itself on her mind.

03 - Mawdryn Undead - 4 Episodes

     Synopsis: The Doctor, Tegan and Nyssa, aboard the TARDIS, find themselves
     caught in the warp ellipse of a starliner that is trapped in time. They find
     a transmat capsule as the source of interference that is trapping the TARDIS,
     and are soon joined by Turlough. Turlough is contacted by the Black Guardian,
     who seeks to kill the Doctor, the Black Guardian offers Turlough passage off
     Earth if he eradicates the Doctor, to which Turlough agrees.

04 - Terminus - 4 Episodes

     Synopsis: The TARDIS, to save itself, has materialized aboard a spaceship
     heading for an unknown destination. It becomes apparent that the spaceship is
     actually a transport carrying sufferers of a leprosy-like disease, to a space
     station named Terminus.

05 - Enlightenment - 4 Episodes

     Synopsis: The Doctor and his companions encounter a group of god-like
     immortals who are racing historical Earth sailing vessels through space,
     crewed by humans they had plucked out of time, in an attempt to win the prize
     of the titular enlightenment.

06 - The King's Demons - 2 Episodes

     Synopsis: The Doctor discovers a shape-shifting robot war weapon, Kamelion,
     posing as King John in 1215 A.D.


07 - 20th Anniversary Special: The Five Doctors - 2 Episodes

     Synopsis: With the Fourth Doctor trapped in a vortex, the First, Second,
     Third and Fifth Doctors, plus various companions, must play the game of
     Rassilon to defeat an unlikely foe.