I had forgotten the name of this episode, “Mawdryn Undead,” and so when I checked out the DVDs of the Black Guardian Trilogy from the library, I remembered the second and third by name but was drawing a blank on what this first one was about. But once I got past that memory failure regarding the name, I realized this was an episode that I absolutely loved when I first saw it in my younger years. After all, if having the Brigadier as a character does good things for an episode, then having him in it twice over all the more so, and the fact that the Brigadier initially seems to have forgotten the Doctor entirely makes for an intriguing mystery.
And in this episode, the Fifth Doctor does what the Third Doctor did so often: reverses the polarity of the neutron flow.
The episode features several inversions of Biblical themes. The Black Guardian had told Turlough that the Doctor is evil. Now that Turlough has had a chance to get to know him, he protests that the Doctor is not as the Black Guardian claimed. The Black Guardian emphasizes that the Doctor’s good is his evil, thus introducing a sort of moral perspectivalism. The Black Guardian also says over the course of the episode that “all things are possible” and “it is finished.” We also encounter immortals who achieved that state using technology stolen from Gallifrey, and now seek death and an escape from their painful existence of perpetual mutation. We also see the Doctor willing to give up his regenerations to save his friends, and one of the immortals comments that “Soon the Doctor will be a time lord no longer. That is his reward for compassion.”
The episode mentions the Doctor having only twelve regenerations. Given that the Doctor was connected to the device which received power from the contact between the two Brigadiers, perhaps it will turn out, should the show wish to continue beyond the 13th Doctor, they could always appeal to this episode having restored the Doctor’s previously lost regenerations, or even super-charging him so that he has more, a number that even he himself has no way of determining. The latter would actually be pretty cool, since it would mean that any time the Doctor was killed, we would be in suspense wondering whether a regeneration would occur or not.