The cops on either side of the tunnel, back in London, spend quite a while getting their heads around that one, and the mean cop guy goes, "Argh!" Because he's like so dedicated to tracking down Christina, because... Who cares? You don't get to know. You're stupid enough not to ask. You've seen an obsessed cop before, on TV, so just substitute whatever his motivation is, and remember to be lazier.
There's a very DW trippy wannabe-cinematic deal with the Doctor eyes staring and then the hot, hot desert day where they've found themselves. The Doctor leads them out into the open air, which they can breathe, and they stand around in the sand, and all the people are scared, and they wander around Tatooine for awhile while aliens in gabardine and gloves watch them and point angrily while clicking and burping.
Back on the bus, a magical black lady is having some kind of fit. Her husband (black, magical level "so-so") wants her to get out of the bus so that she doesn't cook, starting with her brains, and her brains are like, Too late! She goes on and on about generic oracular shit, "the voices are all around us," whatever, you can hear whispering, it's dumb, it turns out the whispering is dead folks. Meanwhile, the Doctor is playing with sand and one of the interchangeable people is like, "Three suns! No way!" They do the whole "That's impossible"/"Yet it's happening" thing, which is obligatory, and the "just like when the Daleks or whatever stole Earth last year" callback thing, and then they talk about the tires and how they're stuck in the sand, as if that's the problem here.
Cops being dicks to each other, and then the Doctor notices Christina putting on sunglasses and thus being more obnoxious than he is, so he sonics the glasses and all is right again. She asks his name, and they do that whole rigmarole, and the only interesting or original line of it is, "You're called The Doctor? That's not a name, that's a psychological condition." Then the Doctor eats some of the sand, evincing a possible second psychological condition.
While the Doctor figures out something troubling about the sand and puts her off as to what it is, one of the interchangeable people comes running up yelling about how the Doctor was babbling about his little machine before, so probably they should go on a witch hunt. The Doctor points out that these interchangeable people are interchangeable with the interchangeable people from "Midnight," an episode I thought was fucking irritating at the time but had a lot going for it, I now realize, by comparison.