The Tardis lands inside an Aztec temple. Barbara steps outside to explore and, after going through a secret trap door, is mistaken by the Aztecs as the reincarnation of an Aztec god, Yetaxa.
As Barbara/Yetaxa's servants, The Doctor, Ian and Susan also become involved in Aztec culture. Ian is made an Aztec warrior, which puts him in conflict with Ixta, who has been in training to become the leader of the Aztec armies.
Susan becomes betrothed to the Perfect Victim, who's to be sacrificed during an upcoming solar eclipse.
Perhaps most interesting is subplot involving The Doctor. The Doctor strikes up a friendship with an older Aztec woman, Cameca that might be more than just friendship. For those that don't like romance in the revived Doctor Who, especially romances involving The Doctor, it's revealing to see that the idea of The Doctor having a romance isn't exactly a new idea. The scenes with The Doctor and Cameca are real highlights to this story.
The plot revolves around the fact that Barbara, who's being perceived by the Aztecs as a goddess, has decided to change history by banning human sacrifices, which put her at odds with The Doctor who says that Barbara can't change history. This also puts Barbara at odds with the High Priest of Sacrifice, Tlotoxl. Tlotoxl accuses Barbara of being a false goddess, but Barbara does have a sympathetic ally in Autloc, the High Priest of Knowledge.
By the end of the story, Barbara has been unable to change much of anything as the sacrifice takes place as planned. But, as the Doctor points out, while she hasn't changed the Aztec culture, she has at least changed the mind and the beliefs of one man, Autloc.
One of the best, if not the best story from Doctor Who's first series and that's saying something since I consider the first Daleks story to be some of the very best that Doctor Who has to offer.
