Unfairly remembered for buggering up UNIT dating, 'Mawdryn Undead' is really a rather good story. It has a very atypical plot for Doctor Who, which makes rare use of the fact that the series can exploit time travel as a plot device. In addition to this, after committing the heinous sin of scripting 'Time-Flight' Peter Grimwade proves surprisingly successful at juggling an old enemy, an old friend, and a new companion.
Firstly, the principle plot concerns Mawdryn, an unfortunate scientist who along with his companions once stole technology from the Time Lords in an attempt to emulate them. Having accidentally condemned themselves to an eternity of perpetual regeneration and mutation, they now seek to find a way of committing suicide, a difficult task for a group of immortals. This plot is used very well for a number of reasons; for one thing, it makes a refreshing change for a story to focus on an opponent of sorts for the Doctor who is not out to gain power, but who wants help to die. Perhaps wisely, the script side steps the moral issues of euthanasia by emphasizing the ghastly state of Mawdryn's existence, which further means that whilst he is suffering as a consequence of his own past actions, he is not an unsympathetic character. He clearly bears no malice towards the Doctor or his companions; he is motivated purely by his desperate search for help. David Collings, one of Doctor Who's finest occasional guest actors, conveys this beautifully, making Mawdryn seem desperate and pathetic, but never frightening, and also eliciting sympathy. The scenes in which he masquerades as the Doctor make for interesting viewing, and Tegan and Nyssa's uncertainty about him is made believable in part by some rather impressive burnt skin makeup. He clearly doesn't look that much like Peter Davison even then, but this gruesome makeup does make it easier to believe that they could mistake him for their friend. Mention of the Doctor brings up another notable element of the Mawdryn plotline, which is the Fifth Doctor's reaction to events. His initial refusal to help Mawdryn is interesting because whilst it's perfectly understandable given the cost, he clearly finds it enormously painful to reject Mawdryn's plea for aid. Even when asked to sacrifice his own life (or rather, future lives) to help someone whose predicament is entirely their own fault, he still finds it hard to turn them away. Davison brings out this facet of the Doctor's personality magnificently, and the Doctor's sympathy for Mawdryn when he first meets him speaks volumes about his character. When he finally agrees to sacrifice his future incarnations to save Tegan and Nyssa, it sums up this most compassionate of Doctors, as he visible struggles with the enormity of the situation that he is facing and still decides to put the lives of his friends before his own. There's also a certain irony inherent in his scenes with Mawdryn; Mawdryn is a thief who stole Gallifreyan technology and thus created his eternal torment. It's easy to condemn him for this and point out, as the Doctor does, that sometimes people need to accept the consequences of their own actions. The irony is that the Doctor also stole Gallifreyan technology, in the form of the TARDIS, and if he hadn't, he would probably have never met Mawdryn.
The second main plot thread concerns new companion Turlough, a companion totally unlike any seen before. Mark Strickson makes an immediate impression in the role, as Turlough proves initially to be selfish, bullying, cruel, and arrogant, as his scenes with Ibbotson attest. As if these unsavoury qualities were not unusual enough in a companion, he soon makes a deal with the Black Guardian to kill the Doctor, and spends the remainder of the story alternating between trying to kill or manipulate the Doctor and trying to weasel out of his agreement. As soon as he agrees to commit murder, he becomes arguably the single untrustworthiest companion to join the TARDIS crew, and as such he's a great character. The Doctor's quick acceptance of his new acquaintance means that he is soon treating Turlough like a trusted friend, which adds an extra edge to proceedings, as Turlough repeatedly turns to the Guardian for instructions. Whilst Turlough will remain in thrall to the Black Guardian for two more stories however, he begins his slow redemption early on; after his initial attempt to kill the Doctor, he realises that the Time Lord is not the creature of evil that his Guardian claimed, and his general dislike of violence soon means that he's looking for less drastic ways to satisfy the Guardian. Strickson is great in the role, playing the arrogant bully with ease in Episode One, and then switching to increasingly panic-stricken coward as he realises that he has (almost literally) made a deal with the devil. His joining of the TARDIS crew at the end holds great promise, as he proves adept at lying and deceit and calmly shakes hands with the Doctor; indeed, it is worth noting that however frantic Turlough gets about his predicament, he always manages to present a calm façade to the TARDIS crew. It's also interesting that Turlough's first appearance in the series involves attempted murder motivated by a selfish desire to escape his exile; lest we forget, in his first televised story, a certain Time Lord also intended to kill a man with a rock in an attempt to get back to his TARDIS
The return of the Black Guardian is rather poorly explained given that he last appeared some years previously in the final scenes of the Key to Time season, but his presence does add an extra dimension to an already multi-layered story, without making it seem too cluttered. The lurking menace of the Guardian works well in conjunction with the use of his new pawn Turlough, and his quiet manipulation of events from behind the scenes is well handled. Despite inexplicably wearing a dead bird on his head, Valentine Dyall's utterly malevolent performance is superb, and he is one of only a few actors who can get away with uttering lines such as "In the name of all that is evil!" without sounding over the top. The Guardian's constant torment of Turlough makes for some great moments, his ability to appear to Turlough anywhere "waking or sleeping" emphasizing the nightmarish situation in which Turlough has placed himself.
The other old face to reappear in 'Mawdryn Undead' is the Brigadier. Whilst the production teams' original plan to bring back Ian Chesterton might have made more sense in light of the school setting, Courtney recaptures his old role with tremendous ease and the Brigadier is very well used. The idea of two temporal aspects of the Brigadier means that the plot makes good use of him, rather than just treating him as a gratuitous guest appearance, and makes for a satisfying addition to the already busy proceedings. Courtney brings to the role an air of dignity that draws on the Brigadier's characterisation in Season Seven, rather than the increasingly ludicrous buffoon that he became under Barry Letts' tenure as producer, and this is partly why he works so well here. Courtney also manages to make Lethbridge-Stewart seem vulnerable whilst maintaining his old character, and his performance in Episode Two as the Doctor tries to discover the cause of his breakdown six years previously is quite touching. The flashback scene as the Brigadier's memory is restored is rather gratuitous but nevertheless strangely satisfying, as the fan in me gets to see clips of old stories; more importantly, the entire scene serves as a reminder of just how strong a friendship developed between the Doctor and the Brigadier. I also hugely appreciate the fact that 'Mawdryn Undead' exploits the two time zone plotline, which is crucial to tying all the disparate elements of the story together in the final scene; with the Black Guardian having stacked the laws of probability against the Doctor, the coincidence of the two Briagdiers touching hands at exactly the right millisecond seems appropriate rather than contrived.
Of the regular cast, I've mentioned Davison already. Given the number of elements vying for screen time already in 'Mawdryn Undead', Sarah Sutton and Janet Fielding get relatively little to do, but when they are used, Grimwade uses them well. By keeping them largely together their different personalities can be exploited; thus, they get to meet Mawdryn and the Brigadier as a pair, which means that Tegan's automatic distrust of Mawdryn contrasts with Nyssa's natural desire to help those in need, and at the same time Nyssa's scientific background is well used throughout the script. Her ability to chip in when the Doctor is spouting explanations results in three way conversations between the TARDIS crew members which makes it seem less obvious that Doctor is explaining the plot to the audience through his companions.
The production of 'Mawdryn Undead' is generally quite impressive; Peter Moffatt's direction is competent if unspectacular, but the location filming greatly benefits the story. The sets too are rather good; those for the school interior nicely match the exterior, and those used for Mawdryn's sets capture the cold grandeur suggested by the script. This contributes to an air of eeriness on board the ship that works particularly well in Episode One, and the incidental score helps. Paddy Kingsland's score is effective throughout, except for some silly music when Turlough is driving the Brigadier's car at the start of Episode One. The costumes used for the mutants are very effective, although exactly how Mawdryn obtains an outfit identical to those of his brethren having been carried into the TARDIS in charred rags is a question left unanswered My only real criticism of 'Mawdryn Undead' is the propensity for technobabble, with talk of warp ellipses and Mawdryn's constant mutation little more than gibberish. The most obvious example if when Tegan and Nyssa become infected; the nature of their infection is very obviously glossed over, with even the Doctor explaining that he doesn't know why he and the Brigadier are immune. Poor explanations mean less technobabble, which is fine, but the fact that travel in the TARDIS seems to affect them differently to Mawdryn once they are infected remains niggling plot hole. But this is a trivial criticism; on the whole, 'Mawdryn Undead' is a rewarding story and one that is deserving of far greater appreciation than it usually gets.