The defining theme of this story is the difficulty - if not the futility - of interfering with a culture you might find wrong from an outsiders' point of view. Or perhaps it would be more accurate to say aspects of a culture, as it is clear that Barbara does admire much about the Aztecs, it's simply the practice of human sacrifice which is abhorrent to her.

Of course the greater problem is the travellers' retrospective knowledge that the Aztec civilisation is doomed. They know that, historically, it will be destroyed after Hernan Cortes arrives in 1519, Cortes having been as revolted by the sacrificial tradition as Barbara. Barbara is thus trying to alter the course of history, not only out of humanitarianism, but in the hope that Cortes will be able to see the better side of Aztec society on his later expedition.

It's important to note, by the way, that this story is clearly taking the view that altering history is not merely morally wrong but actually physically impossible (The Doctor's "What you are trying to do is utterly impossible. I know! Believe me, I know!"). This is in line with David Whitaker's view, also detailed in the Crusaders book's prologue. It is distinct from the assumptions in the series that generally prevailed in the series from The Time Meddler onwards, however I think that to criticise the story on the grounds of stories that hadn't even been written at this stage would be both unfair and irrelevant (and personally I find the thinking behind The Aztecs more convincing than the later approach anyway, much as I enjoy the likes of Time Meddler or Time Warrior).

Hence the importance of it here is that it means that Barbara is struggling desperately to save a civilisation that she knows is doomed, whereas the Doctor already knows she cannot succeed (quite apart form his concerns about whether her attempts to interfere in this way might make life more dangerous for them all before they can find a way back to the TARDIS). I find this a beautifully haunting and moral dilemma, and not one that needs to be cheapened by tiresome continuity arguments.

This central problem is arguably brought to life most vividly and given its sharpest focus in the scene at the start of Episode 2 where Hartnell is bellowing furiously at Jacqueline Hill for having worsened their situation by trying to stop the sacrifice. Susan's having being taken away from them shows how high the stakes are in itself. What's also powerful about the scene is the way Hartnell is able to change gear quickly from ranting anger to contrite and gentle reassurance ("I'm sorry, my dear...I didn't mean to be so harsh with you"), and Hill moves from brittle defiance to abject sorrow, tearfully admitting "you had every right (to be angry)". It's some of the sharpest acting we've seen from both actors in the series so far.

Indeed this must be one of the best uses a female companion ever had in the series. Although Barbara may be misguided, she is acting out of noble motives, and the script credits her with great resourcefulness, intelligence and maturity. For instance, the scene where she uses her ingenuity to stop the fight between Ian and Ixta after Tlotoxl has taunted her to use her heavenly powers, and the clever way she manages to turn Tlotoxl's poison test back on him, help show the character at her strongest. Barbara is conducting a political power struggle here, with supporters (Autloc, and her "servants") and enemies (Tlotoxl and his faction), and having to fight as best she can from her relatively isolated position.

Another key scene is where Ian confronts her after her claim that "Tlotoxl's evil...and he'll make everyone else the same", forcefully telling her "They ARE the same, that's the whole point!" In effect he is showing that Tlotoxl and what he represents isn't simply a removable part of Aztec culture, it is intrinsic to it, and if anything, it is Autloc's well-meaning attempts to take seriously her "prophecy (of) doom" and consider supporting her policy, that is out of place.

Autloc is a character that could be described as triumphant and tragic. Triumphant because he is ultimately liberated from his prior beliefs under Barbara's influence but tragic because this also means a loss of faith in his own traditions and culture - and yet he has nothing to replace them with. All he can do as a last service is to help Cameca to carry out Ian and Susan's escape, but Tlotoxl's claim that he "has gone into the wilderness" seems to imply suicide. Barbara's remorse at the end is easy to understand ("And the one man I had respect for, I deceived"). Keith Pyott gives a very sympathetic portrayal of this gentle elderly man, and he has a good onscreen rapport with Jacqueline Hill. His denunciation when he temporarily loses his belief or support for her ("You are the servants of a false goddess!" is not just painful because of plot reasons - the fact that it means another setback for the regulars - but also because it seems to herald the end of a genuine friendship for unjust reasons). While he does not appear to regain his belief that she is a deity ("I do not know what you are"), his final actions do seem to indicate that he has accepted that Barbara and the others have done him no wrong.

William Hartnell also plays many of his scenes in a far warmer and more affectionate way than has been hitherto usual. It's notable that his Doctor is now calling Ian and Barbara by their Christian names, and is able to joke with and be joked at by the others (Barbara's "You're an old rogue!" and Ian's laughter and "Congratulations!" on hearing of his "engagement"), indicating that the crew have become far closer by now.

On first reading that there had been a Hartnell story where the Doctor "fell in love" to use their inverted commas, but knowing no details, I remember imagining a scenario where the First Doctor had completely fallen under the spell of a pretty woman who was actually a baddie, doting indulgently on her while ignoring everyone's warnings about how dangerous she actually was. It was only on first reading the book that the scenario adopted became clear to me, and it was a nice surprise to find out how tactfully and intelligently it was handled. The Doctor might not actually fall in love as such - his horrified reaction to Cameca accepting his unwitting proposal is rightly seen as a good comic moment - but it's clear he does have great respect, admiration and affection for her. The tenderness he exhibits in his scenes with Cameca helps develop a side of his nature we've normally only glimpsed with his dealings with Susan, and the emotion both feel about their parting is truly touching. It helps that the character easily comes across as worthy of his respect, thanks partly to the writing, and also Margot van Der Burgh's dignified (but not too stilted either) portrayal of her. She does have a sense of fun and goodheartedness, but also a maturity and largeness of spirit.

Ian is put into training as a warrior but it is obvious that he finds the whole thing boring and irritating, and that he looks at Ixta's endless bravado and boasting with nothing but contempt. He has no reason to be interested in leading the Aztec armies and is only there because of appearances (as the Aztecs would not be able to understand why he or "Yetaxa" would want to refuse the "honour"). He all but openly mocks Ixta's militaristic pretensions, and gives the impression of greatly enjoying his chance to get one over him by gripping a pressure point in his body and hence knock him out cold. For Ixta, Ian is a rival for his prestige. For Ian, Ixta is just a nuisance, if a dangerous one. Susan's revulsion for arranged marriages also helps show how out of place the travellers are there, and it is sad to see her, in her desperation, turning on poor Autloc, who is only really trying to do his best, and declaring them all "monsters".

John Ringham gives a very Richard III performance as Tlotoxl, perhaps a little too much so, but it works in context, and he becomes one of the very few, if not the only, adversaries who completely wins the game in the series (save his being denied the chance to have the travellers killed). His carrying out the sacrifice at the end helps to underline Barbara's ultimate failure to change the Aztec civilisation in any tangible way. The most she has achieved is to have destroyed one good man's faith in that society.

Considering the cramped studio bound nature of the story, with its reliance on cloth backdrops and the like, the production values are generally very high indeed, with both good sets and very moody lighting. It probably benefits from having being made in black and white though. The music score might be occasionally a little twee for some tastes, but it is counterbalanced by a lurking ominous tune used for the claustrophobic sarcophagus scenes.

Possibly a bit too worthy and dusty for some, smacking of a keen desire to educate its audience, and at times displaying the mannered style of 60s/70s BBC period dramas, the story is nevertheless well characterised and thought out. Ultimately, it's the most accomplished drama and sophisticated story there had been in the series up to this point.