Caution: potential spoilers.
Planet of the Dead, on BBC One (UK) / BBC America (USA)
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This is the first of four specials airing between series 4 and 5. (Well, actually after The Next Doctor and before series 5... and technically they're all sort of considered part of series 4, I guess.) It begins with a thief named Lady Christina de Souza (Michelle Ryan) stealing some ancient thing from a museum, then making her getaway on a bus, where she meets the Doctor. Then the bus gets sucked through a wormhole to a planet buried in sand. There are a few other passengers on the bus, and the Doctor promises to get them all home. He and Christina meet some flylike aliens who had crashed on the planet, learn of a swarm of flying alien stingraylike creatures, and the Doctor makes a phone call to Earth, where UNIT is watching the wormhole (led by Capt. Erisa Magambo). The Doctor talks to her, as well as a scientist named Malcolm Taylor, who's rather starstruck to be talking with him. Anyway, eventually the bus passengers all get back to Earth. The story's entertaining enough. And in the end, one of the passengers, named Carmen, who is somewhat psychic, has an ominous message for the Doctor (which I won't spoil).
The Waters of Mars, on BBC One (UK) / BBC America (USA)
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The second of four specials airing between series 4 and 5. The Doctor unexpectedly finds himself on Mars, on November 21, 2059. He's captured by the crew of Earth's first base on Mars, led by Captain Adelaide Brooke. When he realizes what date it is, he wants to leave, because history has recorded the destruction of the base and the deaths of the whole crew. And this is not an event he can change, because it's too important to the future; it's a "fixed point in time," unlike most of the points in time he visits. Anyway, the crew gets infected by a virus that turns them, one by one, into water-spewing zombies. In the end, the Doctor tries to defy fate and change history, but his newfound arrogance won't last, in spite of a seeming victory over time. Anyway, the special ends rather abruptly, and I expected it to lead directly into the next special. Not sure what else to say, except that the story had its good points. It was interesting to see some changes in the Doctor....
Between the second and third specials, there was an animated microseries called Dreamland, which probably isn't canonical.
The End of Time, parts 1 and 2, on BBC One (UK) / BBC America (USA)
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The third and fourth of four specials airing between series 4 and 5. And I was wrong to assume part 1 would pick up where "Waters of Mars" ended. When it starts, the Doctor has apparently spent some time roaming time and space, trying his best to enjoy himself and avoid the fate that he knows is coming. There's not much I can say, I'm afraid, about the plot of this story, since I would consider all of it too spoilery of the series in general. I'll just say that Wilfred Mott played an important role. Any other characters seen... the mere mention of them would constitute the spoilers I was talking about. (Though I will spoil it later.) So I'll just say it was a brilliant story, with plenty of action and humor and drama, a touching end... and finally, the Doctor regenerates. Oh, and I guess this is technically 2009's Christmas special, though it's hard for me to think of it as such. (Part one aired on Christmas 2009, and Part two on New Year's Day 2010.)
Followed by series 5