As a result, 26 serials are currently incomplete, with one or more episodes represented only by audio, which in many cases is in addition to clips or still frames.
For commercial release, some episodes have been reconstructed using off-air audio recordings, paired to surviving visuals or newly commissioned animation.
[2] Other sources, such as the Region 1 classic Doctor Who DVD releases, use different numbering schemes, which diverge after the 108th story, The Horns of Nimon (1979–1980).
In his last story, The Tenth Planet, the Doctor gradually grew weaker to the point of collapsing at the end of the fourth episode, leading to his regeneration.
^† Episode is missing This season saw the departure of Carole Ann Ford as Susan Foreman in The Dalek Invasion of Earth, replaced by Maureen O'Brien as Vicki in The Rescue.
William Russell and Jacqueline Hill also departed their roles as Ian Chesterton and Barbara Wright, respectively, in The Chase, replaced by Peter Purves as Steven Taylor.
^† Episode is missing O'Brien departed the role of Vicki in The Myth Makers, replaced by Adrienne Hill as Katarina, and then later by Jackie Lane as Dodo Chaplet.
The Savages marked the final appearance of Steven, and The War Machines introduced companions Ben and Polly.
This season introduced companions Jamie McCrimmon (Frazer Hines) and Victoria Waterfield (Deborah Watling), in The Highlanders and The Evil of the Daleks, respectively.
^† : Episode is missing This season saw the departure of Watling as Victoria, and the debut appearances of Wendy Padbury and Nicholas Courtney as Zoe and Colonel Lethbridge-Stewart, respectively.
This season marked the final appearance of companion Jo Grant and the end of the Doctor's exile on Earth.
He is, to date, the actor who has played the Doctor on television for the longest period of time,[23] having held the role for seven seasons.
During this season, Ian Marter (Harry Sullivan) left after Terror of the Zygons, but returned for a guest appearance in The Android Invasion.
Terror of the Zygons also saw the last semi-regular appearance of Nicholas Courtney (Brigadier Lethbridge-Stewart) who did not return until Season 20 in Mawdryn Undead.
Elisabeth Sladen (Sarah Jane Smith) left the series this season in The Hand of Fear and was replaced by Louise Jameson (Leela) in The Face of Evil .
Castrovalva, together with the previous two serials, The Keeper of Traken and Logopolis, form a trilogy involving the return of the Master.
Mawdryn Undead, Terminus and Enlightenment involve the Black Guardian's plot to force the Doctor's new companion Vislor Turlough to kill the Doctor; they were released individually on VHS and as a set on DVD as parts of The Black Guardian Trilogy.
The season marked the departure of Tegan Jovanka and Vislor Turlough, as well as the introduction of Nicola Bryant as Peri Brown.
Mel Bush left in Dragonfire with recurring character Sabalom Glitz (Tony Selby), and the companion role was taken over by Sophie Aldred as Ace.
The final season continued to push the series towards a darker approach, focusing this time more on Ace's personal life as well as The Doctor's past and manipulations.