Good grief, Maureen O'Brien is a treasure! Nothing against Carole Ann Ford, but this is one of the biggest companion upgrades in series history. They're having so much more fun with the character; I don't know whether it's that they're afraid of losing a second actress in a row if they give her nothing to do but scream, or if "future kid" is just a more accessible concept than "alien kid" and lends itself to more story ideas, but she is firing on all cylinders here. The scene between her and Barbara, where she debates whether to take "medieval" aspirin and Barbara offers to instead put on a mask and dance around the fire to drive the evil spirits out of her body, has to be instantly in my Top Ten Doctor Who Character Moments Ever.
Which is good, because I think an episode like this really needs strong character moments to ground you. The story itself is at this point pretty much incomprehensible; so far we've seen giant ants wander around the TARDIS chittering, a squat grub projecting a light show, Ian's pen teleporting, and Barbara being hypnotized by her bracelet. Even if you take it on faith that there's eventually going to be an explanation that links all this together into a logical narrative, it's certainly not there at this point, and it's pretty much the performance of the regulars that's holding all of it together long enough for things to gel.
Hartnell is keeping things going for his part with sheer manic energy; this is really where you start to see the "giggling with excitement and racing around poking at things" version of the character that's going to become the standard for later incarnations. (I can see Matt Smith, for example, doing a lot of the physical aspects of the Doctor in this episode.) We've left the snappish, rude version of the First Doctor pretty far behind, save for a few comedy moments when he accuses Ian of trying to play a prank on him.
Ian, meanwhile, has kind of hit the limits of his character here. He's still being played well by Russell, but at this point his "man of action" schtick is becoming a bit too one-note. You can start to understand why he's not going to be sticking around much longer; Barbara gets the jokes and the character bits, Vicki gets to be zany and energetic, the Doctor gets to be the lead, and Ian gets to stare sternly into the middle distance with a steely glare and be ready for trouble. It's not a thankless part, but it's one that definitely doesn't get its Christmas cards out until February, if you know what I mean. (And I almost certainly don't.)
Still, this is an episode that's doing what the classic series did well, creating an alien space and then exploring and defining it, even if it is pushing up against the limits of how "alien" the space can be. It says a lot that the cod-futuristic jargon of "atmospheric density jackets", "teaching machines" and other quaint signs of the Future They Thought We Were Going To Have actually seem normal next to the world we're being inserted into. It feels very much like we're starting to push the envelope of just how weird Doctor Who can really get.