The TARDIS arrives in fifteenth century Mexico inside the tomb of one-time Aztec High Priest Yetaxa. The travellers become cut off from the ship when they explore the temple outside and the tomb door closes behind them. Barbara is proclaimed by the High Priest of Knowledge, Autloc, as Yetaxa's divine reincarnation. However, she incurs the enmity of the High Priest of Sacrifice, Tlotoxl, when - against the Doctor's advice - she attempts to use her new-found authority to put an end to the Aztec practice of human sacrifice.

Events reach a climax on the Day of Darkness - the time of a solar eclipse. Ian's unwilling conflict with the Aztecs' 'chosen warrior', Ixta, ends in a fight in which the latter falls to his death from the temple roof. The Doctor manages to reopen the tomb door using a wheel-and-pulley that he has carved (the Aztecs not having mastered the use of the wheel) and the travellers make good their escape.