After skipping over a number of missing episodes we reach The Aztecs, a serial I actually got recommended to me from a few sources. I can see why. It's pretty good.
After arriving in South America a hundred years before the Spanish turn up, Barbara is mistaken for a reincarnation of one of their Gods, and she sees this opportunity to switch them away sacrificing people since that's why the Spanish wiped them out. Obviously the Doctor's against this, but Barbara's determined, but no matter how much she tries things keep springing back to the way history is remembered. And the Doctor again points out that you can't change history. Not One Line!
Which is a bit wrong considering how later things develop, but in some ways I suppose it can be taken as the Doctor just taking a slightly more hard-line version of fixed points in time. If you want to fit it in, in a retcon sort of way. Or he's lying to get Barbara to do what he wants. The Doctor tends to do that. But back then they're still figuring out the rules of Doctor Who. That this line takes place in an historical episode means the point is more obvious. These were a Classic Who type story that more or less disappeared with Troughton. When the show was conceived the BBC decided there would be the space and alien episodes that would teach science, and the historical would obviously cover history. Eventually they realised that kids preferred monsters to something that was more obviously a lesson and the historicals got dropped. Or at least true historicals like this one that are just pure history. An alien menace messing with an historical period was still fair game.
Barbara is very much the focus of this one, and Ian is shuttled off and almost taking command of the Aztec army, which was a bit of a odd storyline. I like how it progressed, with the jealously from his closest rival and previous front runner for the job of General. Ian winning the first fight with a modern understanding of pressure points, and the other guy constantly trying to prove it was a bit of a fluke was fantastic example of using modern day knowledge to get ahead in ancient times. But it just seemed like it was busy work for him while the Doctor and Barbara got on with stuff. The revelation he did national service so therefore he can handle himself is one thing, but being able to win a proper sword and shield fight seems like a step too far. At least he did better than Susan, who was basically written out of the middle two parts. Not that that's a huge problem. The less screaming the better.
Meanwhile, the Doctor's off trying to find out about the secrets of their temple so they can get back to the TARDIS, and during this he meets a lady who takes his fancy. Probably to the chagrin to those Classic Who snobs who moan about New Who and its fascination with the Doctor and romance, here we have the Doctor falling in love, and this is the sixth serial ever. That's saying something. Admittedly his engagement is accidental, but it's pretty clear the two of them share feelings right up to that moment. The Doctor is practically smitten over her. Why is never really made clear, but there's definite feelings there. Yes, he does manipulate her a little to solve things, but then he's the Doctor, of course he manipulates her. That he's so upfront about it is more of a difference. He even keeps the gift with him as a remainder of Cameca, so clearly she meant something to him.
I realise that following this the idea of The Doctor and any hanky-panky in the TARDIS was unheard of in Classic Who (something Tom Baker and Lalla Ward probably ignored), but clearly that wasn't the case at the beginning. I'd argue that Peri jumped on the TARDIS for the the exact same reason Martha did when she met the Tenth Doctor. That Peri's crush regenerated from Peter Davison to Colin Baker might have ended that reasoning, but she certainly had a soft spot for the man who offered her a life in the TARDIS. New Who's just more blatant about it. I'm intrigued where Clara and the Twelfth Doctor's relationship is going to go, as there was definite chemistry between her and Eleven, and now she's in a similar situation as Peri. It's nice to see that there's an argument that the Doctor and romance can be tracked all the way back to Hartnell.