You cannot rewrite history, not one line.
In the fifteenth Century civilisation is hardly civilised, as the Doctor discovers when the TARDIS materialises in an ancient Aztec temple. When Barbara is mistaken for the reincarnation of the Aztec’s High Priest, Yetaxa, she seizes the oppotunity to put an end to the ceremonial rites of human sacrifice and save the Aztecs from self-destruction.
The Doctor must somehow convince Barbara to let history follow its natural course and return to the TARDIS before their real identities are discovered. Can they escape before they become the next blood sacrifices?
Originally transmitted 23/05/64 – 13/06/64
The Aztecs was the fourth episode of season 1.