Into this somewhat unusual setup the TARDIS arrives, caught in a warp ellipse along with some sort of luxury spaceship. The warp ellipse has immobilized the TARDIS, so while Turlough is being manipulated by the Black Guardian into a position near the Doctor, the Doctor himself (now apparently sporting mini-mutton chop sideburns) is trying to locate the ship's transmat beam (last operated six years earlier) so that the TARDIS can leave.
Turlough's introduction to the Doctor is pretty great, by the way; he enters the TARDIS while the others are out, and while he's checking out the TARDIS the Doctor rushes in, before looking slowly at Turlough. The Doctor seems quite willing to trust Turlough; Tegan doesn't, although there doesn't seem to be any reason for her not to. "Nobody from Earth is just going to walk into a transmat capsule," she complains. "As you did into the TARDIS on the Barnet bypass?" Nyssa asks pointedly. Of course, in this case Tegan is right (not that she knows that), and after the Doctor releases the transmat beam down on 1983 Earth, the TARDIS briefly materializes and then is gone again, and while the Doctor tries to figure out what went wrong, Turlough lifts a big rock and prepares to kill the Doctor. "In the name of all that is evil, the Black Guardian orders you to destroy him now!" the Black Guardian cries, which rather ruins the whole "you're doing the universe a favor by killing an evil man" excuse.
Part two is primarily about three things: first, the Doctor's renewed relationship with the Brigadier; second, the badly injured Doctor that Tegan and Nyssa find; and third, the storylines in the two different timelines (1983, where the Doctor is, and 1977, where Tegan and Nyssa are). That first thing is handled rather well; I like the idea of an amnesiac Brigadier who doesn't remember the Doctor or any of his friends or the events he shared (so, what did he think he did at UNIT all those years?), and the way in which the Doctor lifts the Brigadier's mental block is quite lovely, with all those sepia-tinted memories flooding back. I also like the way the subsequent storyline ties in with the third point, with us seeing what Tegan's doing with the 1977 Brigadier (quickly identifiable by the moustache he's sporting that the '83 version lacks) at the same time the 1983 Brigadier remembers what happened and tells the Doctor about it.
That second point is less successful; does anyone buy that that's supposed to be the Doctor? (Mind you, my wife bought it...for about fifteen seconds, by her reckoning.) It's more entertaining how the not-actually-Doctor (as played by David Collings) leaves smears of the body paint he's wearing everywhere -- and if you look, you can see Janet Fielding's hands are just covered with the stuff after she drags him inside the TARDIS. But even though it's less successful, it ends up being the driving force behind much of the episode, and while we learn that he's actually Mawdryn, not the Doctor (thus explaining the story's title), there's also some intriguing stuff about him becoming a Time Lord. And his appearance at the end of the episode, when the '77 Brig enters the TARDIS, is quite horrific, with his pulsing exposed brain...
122 Remember, in England a public school is, somewhat confusingly, a type of private school.