Wow. Two good episodes in a row. That's something of a record in this season of Doctor Who. So we start off with two people who are something of an anomaly in contemporary television. They are, in fact, not particularly attractive. These two not particularly attractive people (James Corden as Craig, Daisy Haggard as Sophie) are obviously in love with each other, but are worried about ruining the friendship and all that jazz, and so aren't ever intending to do anything about it. That is, until the Doctor gets stranded in London without his trusty time machine, moves into Craig's flat, and in Matt Smith's usual crazy way, manages to cause chaos. But, you know, the good type of chaos, where everything turns out alright by the end. The two not particularly attractive people reveal their love for each other, and get straight to ruining the hell out of their friendship. Happiness and smiles all round.
This being Doctor Who, there are, of course, some horrible aliens lurking around bent on killing everyone on the Earth. But fortunately, the episode doesn't really pay much attention to them, and goes about telling the sweet and charming story of Sophie and Craig, leaving the aliens to just a few small (and legitimately creepy) cameos right up until the climax.
This was the correct decision. The human characters presented here are far more interesting than any aliens we've seen this season (with the exception of the Doctor, of course.) And by keeping the aliens firmly in the background, they actually manage to successfully build suspense about them, something which basically every other episode this season has resolutely failed to do.
This episode's only real problem was the basically total removal of Amy Pond from the storyline. I assume that this was for budget and scheduling reasons - in previous seasons there has always had to be one episode that featured the Doctor and his companion barely at all (the excellent Blink, the reasonably good Love and Monsters). My guess is that Stephen Moffat decided to get around this problem by having the cast split up into two seperate groups which could be filmed simultaneously, and because Amy wasn't in it all that much, she could then go on to film some of her scenes from other episodes. This seems like a reasonable plan, and the episode wouldn't have made any sense with Amy standing around, screwing up the Doctor's attempts to screw up Craig's attempts to screw up his life, but still. I would've liked a bit more Amy. Other than that, though, a good, solid episode.