Mexico, the early 15th century.The TARDIS arrives in the tomb of former Aztec High Priest Yetaxa. The travellers become cut off from the ship when they go and explore the temple outside. Barbara is acclaimed by the High Priest of Knowledge, Autloc, as Yetaxa's divine reincarnation. However, she incurs the wrath of the High Priest of Sacrifice, Tlotoxl, when she attempts to use her new-found authority to stop the Aztec practice of human sacrifice. Can Barbara change the course of history and save the Aztecs? Can the time travellers find a way back into the tomb? Or will they fall victim to Tlotoxl's altar of sacrifice?