This is the first story that really tackles the fact that as time travellers the Doctor and his companions do have the power to change things in the past and potentially make a real mess of things. Something that the Doctor warns against on several occasions saying that they “Can’t rewrite history. Not one line.” Something that Matt Smith’s Doctor seemed to completely forget.

The TARDIS lands in the back of an Aztec tomb and Barbara is initially overjoyed. She tells Susan that the Aztecs are her specialist subject and goes through a few of the objects, putting on an intricate bracelet. The two girls find a hidden door and while Susan goes back to tell the Doctor and Ian what they’ve discovered, Barbara goes further through the tomb to explore. She is discovered by an Aztec priest but because of the bracelet she’s found, they believe her to be a manifestation of the High Priest Yetaxa and when the others join them, say that they are clearly her servants. Something they have to play along with.
Barbara is in her element here. She wants to help the Aztecs by changing the course of history but of course things don’t work out that way. This story is very good in terms of the characters. We really get to see Barbara come into her own. This is what now would be known as a companion-centric or Doctor-lite episode. The Doctor’s story in this is a complete side plot where he sits in a very pretty garden, drinks hot chocolate and accidentally gets engaged to a woman who has become rather smitten with him in the process.
For a story in the 1960s to revolve around a woman being in charge and doing what she thinks is best for these people is an amazing thing, even though it doesn’t all go according to Barbara’s plans. She wants to change history in order to make things better for the Aztecs, knowing what will happen to them if they continue the way that they are. Even though the Doctor repeatedly warns her against trying to change time, her compassionate side compels her to try anyway, even though all she does seem to do is accidentally ruin the life of Autloc – The High Priest of Knowledge – by convincing him that they do not need to perform human sacrifices to their gods.

While Susan doesn’t do very much in this story other than study, Ian on the other hand is off becoming the next Aztec warrior leader.
He has a rivalry with the current leader Ixta, who sees him as a real threat after Ian uses his knowledge of anatomy to render Ixta unconscious by pressing on one of his pressure points. Ixta tries to use unscrupulous means in order to beat Ian. However in their final fight on top of the tomb, Ian gains the upper hand and Ixta is thrown from the tomb. Ian throws a warrior Aztec in his prime off a pyramid. And thus, Ian is awesome!

The crew have managed to find a way back to the TARDIS thanks to Ixta’s efforts to kill Ian by locking him in there earlier and they decide that it’s probably best to beat a hasty retreat from Aztec era Earth and on to their next adventure.