The basic idea here is that the Doctor turns up on Mars, and wanders over to visit the colony. It turns out that this is in fact the first human colony on Mars, but he's visited on the day that it's going to be destroyed. He seemed surprised by this, so presumably he didn't intend to visit that day. So, either the TARDIS is acting up again, or he just hit the "I feel lucky" button and didn't bother checking the display when he landed.
Initially, it looked as though the colony was destroyed by a meteor crashing into Mars, but later on they revealed that the base commander used the self-destruct option to detonate a nuclear bomb. (Am I just hallucinating, or did anyone else see the rock crash into the planet early on?)
The key theme of this episode was that this was a pivotal point in history, so the Doctor couldn't interfere. I've mentioned this idea before (in 2006 and 2008), and they've gradually been making it more explicit: basically, the Doctor isn't as reckless as he looks, because he knows what changes he can get away with. That's fair enough, but I think there's a bit of wiggle room here. In fact, about halfway through the episode I said "No, I don't accept this." Future history shows that the colony was destroyed, but presumably no bodies were found. So, why not use his TARDIS to take everyone away before the explosion, then drop them off in the future (after they've left their legacy) or on another planet (where they won't be able to contact Earth)? The Doctor mentioned Pompeii, and I had the same reaction to that episode: if people are doomed to die, that's a pretty good opportunity for him to recruit new companions. Speaking of companions, it was Donna who nagged him into rescuing Caecilius, so I wondered whether that was the deeper point of this episode, i.e. showing that he needs someone around to influence him, otherwise he'll just stand back and watch the tragedy unfold.
As it turned out, he did do something similar, although he sent a remote-controlled robot off to fetch the TARDIS and bring it back; I was thinking that he could just walk there after he left the base and then bring it back himself. No need to hurry, since it's a time machine: as long as he's far enough away not to die in the explosion, he could then go back to just beforehand, without any risk of meeting himself. This would have enabled him to save extra people, rather than leaving them to die, but never mind.
As a related issue, I don't think the dramatic escape quite worked. Basically, we saw the self-destruct clock tick down to one second left, while everyone was still outside the TARDIS (and a couple of people were sitting on the ground). Then we saw the base explode, then we saw the TARDIS materialise back on Earth. So, in that single second, all four of them were able to get into the TARDIS, the Doctor was able to program the controls, and it was able to dematerialise? No. I'm sorry, but that's just stretching credibility too far. Again, this is where my proposed method would have worked better, since we could have seen the "original history", with the remaining crew alone in the room watching the timer hit zero, then the explosion, and then seen the revised history when the Doctor goes back to change it. That way, you get the same dramatic tension ("oh no, they're all dead!"), which would actually seem more plausible when the lead character is absent, but you also get the last minute rescue.
Once they did all leave Mars, why go to Earth in the "present day" (2059)? That's going to mess up history a lot more than taking them to another place/time. Then again, as I've said before, I sympathise with Johnny Maxwell's attitude: "Everything we do affects the future! It always has. It always will." It was interesting to see the Doctor taking a Judge Dredd attitude towards the timeline ("I am the law!"), and it was fairly clear that this was intended as a character flaw rather than a happy ending.
Ending on a positive note, I liked the way that the characters tried to give a decent medical description of what had happened to the infected people (e.g. cracked skin around the mouth) rather than just saying "They're zombies!" And I definitely agree with the Doctor that cycling around the base would be a good idea. Bromptons in space!
Anyway, I'm looking forward to the next episode (on Christmas Day), and there's not too long to wait for that now.