Wow. I think this episode may contain the single worst scene in Doctor Who history.
When Susan makes the somewhat reckless decision to entrust herself to the Sensorites, the Doctor and Ian and Barbara go after her. Does the Doctor try to convince her to discuss the issue rationally instead of making a unilateral decision? Does he express his concern for her safety? Does he buggery. The Doctor treats her like a cranky toddler who needs a time out, shouting at her in a horribly condescending and patronizing fashion. Honestly, it feels like the only reason he stops shy of spanking her and making her stand in the corner is that Susan folds so quickly on the one independent decision she's made since the start of the series that it makes you feel like she's been browbeaten to the point where she's afraid to make choices of her own. Anyone who thinks the Doctor is an arrogant, rude, sexist git to his companions in the new series needs to watch this scene, so at least they'll know he's mellowed some.
Other than that, this episode is surprisingly good. I mean, it pretty much does mark the end of any kind of menace the Sensorites have...once the Doctor defeats an enemy by turning down the lights and collecting the weapons they dropped in sheer blind panic over being in the darkness, you have to admit that they're probably never going to reach "iconic monster" status. But it's also where they start being treated like actual characters, which is a vast improvement and something Doctor Who has never really done often enough. Non-human aliens usually get relegated to "spooky spooky scary scary" status, but here the Sensorites are given a government, a society, and even some political intrigue to deal with.
And the political intrigue is actually intriguing. You have the First Elder, who wants to open up his society to outsiders; you have the Second Elder, who's doubtful about the idea but who's willing to work within the system to express those doubts. And you have the City Administrator, who's a straight-up xenophobe willing to undermine his own leader and the underlying structure of his society to "preserve" it from the alien freaks, and who's recruiting others to his cause. That's some pretty decent depth for a show that usually depicts alien cultures as "despotic tyrants" and "heroic rebels". Oh, and there's a plague going on that's been stymieing their efforts to cure it and decimating their citizenry. (And which doesn't strike the Elders, which may go some way to explaining why the City Administrator's not feeling too charitable towards them.) Admittedly, "mysterious plague" tends to come fitted as standard in Doctor Who stories, but still, it's some welcome complexity in a story that's been struggling to fill its six parts.
Oh, and Stephen Dartnell continues to impress as John, and Ilona Rogers gets in a good scene as Carol, and we're finally rid of Maitland and his plywood performance. Yes, this is picking up nicely even before Ian collapses at the episode's end. Doesn't mean I don't want to give the Doctor a good slap, but I still think they're finally hitting some kind of stride.