This is a four-part series, first broadcast between 23rd May - 13th June 1964. The serial was written by John Lucarotti, produced by Verity Lambert and directed by John Crockett. The script editor was David Whitaker.
When the Tardis lands the Doctor and his companions Susan, Ian and Barbara find themselves among the ancient Aztecs. In this story Barbara is venerated as a goddess, even if in the end her followers lose faith and she fails to get the Aztecs to abolish human sacrifice, and both the Doctor and Susan get engaged though neither ends up married.
The story is a common one in children's fiction: present-day characters returning to some historical period and changing - or, as the case may be, failing to change - history.
To a modern audience, this series' main flaw would be its age. It's black and white, it's low on action and high on dialogue (though the action that is there is quite effective), it's got people wearing a 1960s designer's interpretation of ancient Aztec garb and it uses somewhat stilted English to denote the language of the Aztecs.
However, if you're willing to look past all that, you'll find a story that is well-written and well-paced and that gives us some memorable, well-rounded characters.
Not a must-see serial, but quite enjoyable.