When the First Doctor, Susan, Ian and Barbara arrive accidentally in ancient Mexico, Barbara is mistaken for a god by the Aztecs. Horrified by the barbarous practice of human sacrifice which she is forced to observe, she is determined to use her power to prevent such atrocities from ever happening again. The Doctor must use every means in his power to prevent her from interfering with the natural course of history, but he himself is occupied by other matters - such as the rather attractive Cameca...