The Doctor: My fiancée.
Ian: I see ... your what?
The Doctor: Yes, I made some cocoa and got engaged.
- 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.
- Aztec Mythology
- The Bad Guy Wins: The villain 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 episode 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.
- Disney Villain Death: Ian throws Ixta off the top of the temple.
- Driven to Suicide: The intended victim of the sacrifice Barbara interrupts, encouraged by Tlotoxl.
- Evil Versus Evil: Cortes' conquest of the Aztecs. Barbara wanted to prevent the Evil Versus Evil aspect at least.
- 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.
- God Guise
- 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 is able to escape the tomb by using a wheel, which the Azrecs did not have. The Doctor takes the wheel with him so history will not be changed.
- Large Ham: Tlotoxl. Who knew Richard III was in fact an Aztec high priest?
- Manipulative Bastard: Tlotoxl
- 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.
- Punch Clock Villain: Tlotoxl.
- 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
- Shown Their Work: Notably, Mayincatec is averted, as the screenwriters evidently did a bit of research.
- 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").
- The X of Y: All four episode titles follow this format.
- You Can't Fight Fate: "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 Aztecs civilisation being destroyed is a fixed point in time.