The Doctor: My fiancée.
Ian: I see ... your what?!
The Doctor: Yes, I made some cocoa and got engaged.
The one where Barbara steals jewellery.
It's time for another historical story, as the TARDIS materialises inside an Aztec tomb. The Aztec era happens to be one of Barbara's specialties, and she loves the tomb's shiny trinkets so much that she immediately puts one on. Upon emerging from the tomb wearing the snake bracelet, Barbara is hailed as the reincarnation of the Goddess Yetaxa. Oh, and the door to the tomb shuts itself behind them.
Despite the Doctor's very stern warnings, Barbara seizes the opportunity to influence history. Her intentions are good (she decrees an end to the practice of Human Sacrifice), but she instantly makes one of the high priests suspicious of her alleged divinity and pitches the other one headlong into a crisis of faith. Barbara finds that even a goddess can't effect social change by fiat.
Meanwhile, the Doctor is told to rest in the serene gardens together with the rest of the old folks. He befriends Cameca, an very sweet Aztec woman, who turns out to know the son of the tomb's architect. Seeing a way out, the Doctor spends more and more time with her... and accidentally gets engaged when he makes a cup of hot cocoa.
Ian, off on his own, actually meets the son of the tomb's architect, who immediately takes exception to him — having been the champion of the Gods until Ian arrived. Ian is forced to fight him in judicial combat, and the Doctor, hoping to get his hands on the tomb's blueprints, unwittingly helps the guy (not knowing that Ian will be his opponent). Luckily, Ian can easily hold his own in combat.
In the middle of all that, Susan is sent off to a seminary for proper education, and gets into an Arranged Marriage as part of a plot by Tlotoxl to trap and discredit Barbara.
In the end, the Doctor bids Cameca farewell, lashes up a pulley wheel to open the massive tomb door and let them back in to escape in the TARDIS. Barbara didn't convince anyone to stop human sacrifices, but her priest friend at least managed to separate himself from Aztec society and became a hermit instead.
- Accidental Proposal: Offering to make a cup of hot cocoa for a woman is considered a marriage proposal by the Aztecs.
- Arranged Marriage: Susan is almost forced to marry a guy who would become a Human Sacrifice three days later. He plans to spend his final few days with a nice young wife.
- Author Appeal: Having lived in Mexico, writer John Lucarotti was fascinated by the Aztec civilisation. He described them as "a highly civilised and cultured race", and was particularly fascinated by their obsession with human sacrifice.
- The Bad Guy Wins: Tlotoxl gets rid of the pesky false god, his rival, and he gets to keep sacrificing. For the time being at least. The Aztecs will still be massacred and enslaved by the Spanish and the practice of human sacrifice will cause future generations to look down upon the Aztecs as mindless savages. In other words: this story was a lose-lose situation.
- Better Manhandle the Murder Weapon / The Corpse Stops Here: Ian with the attack on Autloc (although in this case Autloc isn't actually dead)
- Bookcase Passage: The innocuous-looking stone in the garden.
- Breaking the Fourth Wall: Slightly at the end of the first episode, as the evil high priest looks straight at the audience when he vows to destroy Barbara.
- Dated History: Barbara's argument that stopping the Aztecs performing human sacrifice will prevent their conquest by Cortes is pretty diametrically opposed to how modern historians view the colonisation of South America by Europeans. In particular, the question of what happened to all the other civilisations the Spanish wiped out, most of whom had nothing to do with Human Sacrifice, never comes up. However considering Barbara is from the era this story came out it is hardly surprising she holds 1960s views.
- Disney Villain Death: Ian throws Ixta off the top of the temple.
- Downer Ending: The story ends with the TARDIS crew leaving Aztec Society knowing it will soon be destroyed, the High Priest of Sacrifice ends up in control and the best that they did was cause the man who was opposing the High Priest of Human Sacrifice to leave society for life as a hermit.
- Driven to Suicide: The intended victim of the sacrifice Barbara interrupts, encouraged by Tlotoxl.
- Early Instalment Weirdness: The Doctor has a romantic subplot with a guest character, something which for most of the original series would be unthinkable.
- Everyone Calls Him "Barkeep": The 'perfect victim' chosen to be the next human sacrifice is only ever known as 'Perfect Victim'.
- Evil vs. Evil: Cortes' conquest of the Aztecs. Barbara wanted to prevent the Evil vs. Evil aspect at least.
- Actually a case of Dated History as, at the time the serial aired, historians viewed Cortes as a peaceful science-minded explorer who was driven, reluctantly, to conquer the Aztecs only because of his horror at the evil of human sacrifice. So Barbara thought it was evil versus good and that she could avert the whole thing by talking the evil out of it. The DVD release includes a BBC documentary of the time, unironically presenting this older view of Cortes.
- Family-Unfriendly Violence: Ian tries to intimidate a bunch of Aztec warriors by taking them aside with a knife and saying in a very calm, terrifying and unflinching voice that he could stab them to death at any time, in any place, exactly when they least expect it. Ian is normally a Reasonable Authority Figure who makes dad jokes, not a Terror Hero.
- Faux Affably Evil: Ixta's treatment of Ian comes across as this, although thanks to Values Dissonance he thinks he's behaving like a Worthy Opponent.
- Girl of the Week: It's pretty clear from the script and acting that there is some romantic tension between the Doctor and Cameca, even though the proposal was accidental.
- A God Am I: While pretending to be a god, Barbara believes herself capable of rewriting Aztec culture and history itself, despite the Doctor telling her otherwise.
- God Guise: Barbara is mistaken for the Aztec deity Yetaxa and tries to use her position to change the Aztec Empire.
- Grave Robbing: Implied as the reason why Ixta's father put a secret passage into Yetaxa's tomb.
- Human Sacrifice
- Inventing the Wheel: The TARDIS crew escapes the tomb by using a wheel, which the Aztecs did not have. The Doctor takes the wheel with him so history will not be changed.
- Jerkass Has a Point: Barbara is a false goddess, so Tlotoxl has that bit right.
- Kleptomaniac Hero: Barbara's love of history doesn't prevent her from trying on shiny things she finds in graves.
- Large Ham: Tlotoxl. Who knew Richard III was in fact an Aztec high priest?
- Manipulative Bastard: Tlotoxl.
- Mighty Whitey: When Ian is forced to compete for command of the armies of the Aztecs with the best soldier in the empire. Rather than realistically being portrayed as out of his league, he manages to beat the Aztec warrior with one thumb and later the Aztec has to resort to poisoning him to stand a chance of beating him. Somewhat justified in that British National Service laws of the time would have meant Ian served for a minimum of 18 months in the military.
- Nice Hat: Almost everyone in the story gets to wear silly hats.
- Oh, Crap!: The Doctor when he realizes he's engaged.
- Posthumous Character: The un-named tomb-builder who was Ixta's father and Cameca's lover.
- Pressure Point: How Ian wins his battle against Ixta, only using his thumb.
- Protagonist-Centred Morality: Tlotoxl, of course, is quite right. All his actions (extreme though some of them may be) seem to be in the genuine interests of stopping his people from being deceived by the false goddess. Strictly speaking, it's the time travellers who are in the wrong, the odd attempted murder notwithstanding.
- Punch-Clock Villain: Tlotoxl.
- Randomized Title Screen: The DVD issue of the story shows a title screen with a random pick from four lines by different members of the cast in character.
- Reasonable Authority Figure: Autloc
- Right for the Wrong Reasons: Tlotoxl says they will be destroyed if they follow "Yetaxa's" command to abolish Human Sacrifice, but it has less to do with the gods not being appeased, and more to do with the Clock Roaches.
- Right Makes Might
- Ripple Effect-Proof Memory: The story argues that you can't change the past anyway so the situation would never come up in the first place.
- Shown Their Work:
- Notably, Mayincatec is averted, as the writer John Lucarotti evidently did a bit of research.
- Designer Barry Newbery found the serial difficult for research, due to the limited information on the Aztecs at the time. Researching using books provided by The BBC, he designed the tomb door based on the "comic book" style of Aztec design. He had also watched a documentary about Aztec archaeology on ITV and was concerned that a larger studio would be required for production.
- Unbuilt Trope: Barbara tries and fails to save Aztec civilisation by ending human sacrifice. The ending is, at best, bittersweet, in which the High Priest of Sacrifice ending up in control and the only consolation is that the High Priest of Art leaves society to meditate on his faith.
- The Unpronounceable: The cast members never seemed to have reached a consensus on how "Tlotoxl" is actually said, although nobody manages to get it right (it would've been more like "Tlo-tosh-l").
- Unwanted Rescue: When Barbara acts to halt the Human Sacrifice, the chosen sacrifice instead throws himself off the top of the pyramid to his death: being chosen as a sacrifice being a great honour in Aztec society.
- The X of Y: All four episode titles follow this format.
- You Can't Fight Fate: The first serial in the show to explore the limitations and ramifications of time travel and interfering with the past. As the Doctor himself says, "you can't rewrite history, Barbara; not one line" (Later stories show that it's actually closer to "you can change history, but it's a really bad idea to do so")!
- In "Defining the First Doctor" Steven Moffat says that as the Doctor is starting out he hasn't quite got the rules of time travel and that we can assume Aztec civilisation being destroyed is a fixed point in time.