Mawdryn Undead is the third serial of the 20th season in the British science fiction television series Doctor Who, which was originally broadcast in four twice weekly parts from 1 February to 9 February 1983. The serial was the first of three loosely connected serials known as the Black Guardian Trilogy, and introduced Mark Strickson as a new companion, Vislor Turlough, as well as reintroducing Nicholas Courtney as Brigadier Lethbridge-Stewart. The character had not been seen on the series since the Fourth Doctor serial Terror of the Zygons almost eight years earlier.
In 1983, Brigadier Lethbridge-Stewart has retired from UNIT, and teaches mathematics at Brendon Public School. One of his students, Turlough, a stranded alien Trion posing as a human, takes the Brigadier's classic car for a joyride but ends up in a crash. While unconscious, Turlough is contacted by the Black Guardian, who seeks to kill the Doctor for his interference in acquiring The Key to Time. The Black Guardian offers Turlough passage off Earth if he kills the Doctor, to which Turlough agrees, and is given a communication device through which the Black Guardian gives him orders.
The Doctor, Tegan and Nyssa, aboard the TARDIS, find themselves caught in the warp ellipse of a starliner that is trapped in time. Materialising aboard the ship, they find a transmat capsule as the source of interference that is trapping the TARDIS, and are soon joined by Turlough, following the Black Guardian's orders. The Doctor takes Turlough with him to Earth to locate the other end of the transmat, leaving Tegan and Nyssa aboard the TARDIS on the ship. After the Doctor corrects the transmat device on Earth, the TARDIS attempts to materialise but soon vanishes. Without any idea where to look, the Doctor talks to the Brigadier at the school, surprised that the Brigadier has forgotten their past histories due to some type of trauma. However, on mention of finding Tegan and the TARDIS, the Brigadier recalls seeing the TARDIS in 1977. The Doctor attempts to coax the details of these events from the Brigadier.
Meanwhile, in 1977, Nyssa and Tegan leave the TARDIS, and discover the other end of the transmat capsule near the school. Inside is a horribly disfigured alien humanoid, which they believe may be the Doctor. They find local help nearby from the Brigadier, still on active duty with UNIT. The alien, still pretending to be the Doctor, convinces the three to return with him to the TARDIS and return to the spaceliner where a remedy for his situation may be found. In 1983, the Doctor tracks the TARDIS' movements and realises it returned to the liner. He, Turlough, and the older Brigadier return to the liner via the transmat capsule. Due to the nature of the warp ellipse, the Doctor rejoins his allies, but on discovery that two versions of the Brigadier are aboard, cautions his companions to keep them apart, lest the resulting energy discharge prove catastrophic.
Ultimately, the Doctor learns the truth of the liner's crew, that they are several scientists that attempted to discover the Time Lord secret of regeneration, but ended up as disfigured creatures, unable to die. The alien that Tegan and Nyssa met reveals himself as Mawdryn, and tries to convince the Doctor to give up his remaining regenerations to kill Mawdryn and his fellow sufferers. The Doctor tries to leave with his companions in the TARDIS, but finds that Tegan and Nyssa are now suffering the same condition as Mawdryn, and de-age rapidly while the TARDIS is in the Time Vortex. The Doctor realises he has no choice, and prepares to give up his remaining regenerations to not only kill the scientists but to also restore Tegan and Nyssa to normal.
During this time, the two Brigadiers have been purposely kept apart by Turlough and Mawdryn and his crew. Mawdryn even attempts to send the younger Brigadier back to Earth to avoid him touching his other self, but the transmat capsule fails and returns to the ship. As the Doctor is about to engage the device to take his regenerations, the two Brigadiers finally meet and reach out to touch the other. The resulting temporal energy release occurs at exactly the right time to perform the same actions that the Doctor's sacrifice would have done: Mawdryn and the other scientists are freed of their undead existence, and Tegan and Nyssa return to their proper age. The younger Brigadier suffers a trauma that causes him to forget the Doctor and these events, until later reminded of them in 1983. The Doctor evacuates everyone to the TARDIS before the liner self-destructs, and then returns both Brigadiers to their respective time streams on Earth. The Doctor accepts Turlough's request to join his crew, unaware of his influence by the Black Guardian.
All of the stories during Season 20 featured enemies from the Doctor's past. The past enemy for this and the next two serials was the Black Guardian, who last faced the fourth incarnation of the Doctor at the conclusion of The Key to Time saga in The Armageddon Factor (1979). The Black Guardian Trilogy continues in the following serial, Terminus. It was also the first Fifth Doctor episode to star Nicholas Courtney as The Brigadier. During the Brigadier's flashback he sees Yeti (The Web of Fear), Cybermen (The Invasion), the Second Doctor (The Three Doctors), the Axons (The Claws of Axos), Daleks (Day of the Daleks), the Third Doctor (Spearhead from Space), the First Doctor (The Three Doctors), the K1 robot (Robot), a Zygon (Terror of the Zygons), the Fourth Doctor, and finally himself from The Three Doctors. All of the clips were shown in sepia-tinted black & white.
Mawdryn Undead also makes the first explicit statement in the series that the current Doctor is the fifth incarnation. The Doctor clearly states that he has eight incarnations left after his present one, confirming that there were no earlier incarnations before the televised first, played by William Hartnell.
The Doctor cites the "Blinovitch Limitation Effect" as the reason for the temporal energy discharge resulting from the meeting of the two Brigadiers; this was first mentioned in the Third Doctor serial Day of the Daleks. However, the effect does not seem to apply to Time Lords, or it can at least be mitigated, as the Doctor has met his prior incarnations on several occasions (the difference between incarnations may itself be an explanation). In the fourth episode, the Doctor says he might try to "reverse the polarity of the neutron flow", a phrase often associated with the Third Doctor.
The Doctor refers to several people that he worked with in UNIT, namely Sergeant Benton, Harry Sullivan, Jo Grant, Sarah Jane Smith and Liz Shaw. In a rare case of an explicit calendar date being applied to the UNIT timeline, the Brigadier indicates that Benton left UNIT in 1979 and became a used-car salesman, and that Sullivan had been seconded into doing secret government work at some point prior to 1983. The episode also establishes that the Brigadier left UNIT in 1976 and became a teacher, although the serials The Five Doctors and Battlefield establish that he later re-established his relationship with the organization. (See UNIT dating controversy.)
Serial details by episode
||1 February 1983
||2 February 1983
||8 February 1983
||9 February 1983
Mawdryn Undead was a replacement for an earlier script, The Song of the Space Whale, by Pat Mills. That script fell through when Mills and script editor Eric Saward could not agree on certain elements of the story. Instead, Peter Grimwade quickly produced Mawdryn Undead to fill the gap in the production schedule and provide the first instalment of the Black Guardian Trilogy. The Song of the Space Whale was later renamed The Song of Megaptera and made into an audio drama by Big Finish Productions for their Doctor Who The Lost Stories range.
Cast notes 
The original intent of the production team was for the character of Ian Chesterton, one of the original regulars from the series' first two seasons from 1963–1965, to return for a guest appearance in this story; hence the school setting, as Chesterton was a science teacher, and the Brigadier's being issued with another TARDIS homing device. However, actor William Russell proved to be unavailable. Some consideration was given to using instead the character of Harry Sullivan, who was a regular in the programme for a season in the mid-1970s, before the return of Lethbridge-Stewart was eventually decided upon.
David Collings, who played Mawdryn, also appeared in the Fourth Doctor serials Revenge of the Cybermen as Vorus and The Robots of Death as Poul, and would himself play an alternate Doctor in Big Finish Productions' Doctor Who Unbound audio play, Full Fathom Five. Angus MacKay previously played Borusa in The Deadly Assassin. John Nathan-Turner felt that Mark Strickson's blond hair didn't stand out well enough from Peter Davison's blond hair. He initially asked Strickson to shave his head, but when Strickson declined, Turner decided that Strickson's hair should be dyed red.
In print 
A novelisation of this serial, written by Peter Grimwade, was published by Target Books in August 1983.
VHS and DVD releases 
Mawdryn Undead was released on VHS in November 1992. It was released on DVD as part of the Black Guardian Trilogy on 10 August 2009 (Region 2), with a commentary by Peter Davison, Mark Strickson, Nicholas Courtney and Eric Saward and an option to view the story with new CGI effects. The serial was also released in issue 50 of the Doctor Who DVD Files, published 1 December 2010.
External links 
Mawdryn Undead at the Doctor Who Reference Guide
Mawdryn Undead reviews at Outpost Gallifrey
Target novelisation