Ian in Danger

The TARDIS arrives in fifteenth century Mexico inside the tomb of one-time Aztec High Priest Yetaxa. When the four travellers leave the tomb, the door swings shut behind them and they find that they are unable to open it from the outside and so cut off from the TARDIS.

They soon encounter Autloc, the High Priest of Knowledge, and Tlotoxl, the High Priest of Sacrifice. Seeing that Barbara Wright is wearing Yetaxa's bracelet, which she found in the tomb, Autloc proclaims her to be a divine reincarnation. While Susan stays with Barbara as her handmaiden, Ian Chesterton is persuaded to represent the gods as leader of the Aztec army while The Doctor joins the Aztec elders in the Garden of Peace.

In her new position of power Barbara sees her chance to bring an end to human sacrifice. She sees the good side of Aztec culture manifested in Autloc, the High Priest of Knowledge, and the gruesome side embodied in 'the local butcher' High Priest of Sacrifice, Tlotoxl.

The Doctor

As somewhat of an expert on this period, Barbara sees how advanced their culture really is and believes that if sacrifice were abolished, they would be spared destruction at the hands of the Spanish. The Doctor's urgent warnings that Barbara cannot change history fall on deaf ears, much to The Doctor's fury.

However, Barbara incurs the enmity of the bloodthirsty Tlotoxl when he begins to suspect Barbara is not Yetaxa returned, especially as she attempts to ban the human sacrifices. He therefore sets a series of elaborate traps for her and her companions so as to prove that Barbara is not Yetaxa.

Barbara in Yetaxa's Headdress

These include Ian being compelled into fighting the strongest warrior, Ixta, to prove his ability to command the Aztec forces. Thus Ixta develops a grudge against Ian and this is used by Tlotoxl to try and then poison Barbara. He plans that the death of Barbara following consumption of the poison would prove she is not immortal and therefore not a god.

Ian though is able to warn her and so Barbara refuses to drink the poison. Susan, meanwhile, causes an outcry when she refuses to become the wife of the Perfect Victim before he is sacrificed to the gods. While The Doctor, who knows little of Aztec customs, has become accidentally engaged to an Aztec woman named Cameca after they shared a cup of cocoa.

Events though reach a climax on the Day of Darkness at 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.

Tlotoxl

With the help of Cameca, The Doctor and Ian find a way to re-enter the tomb by a secret entrance. After negotiating a treacherous tunnel, in which he is almost drowned, Ian is able to re-enter the tomb, by a secret door, so providing a means for them to escape.

Despite her efforts Barbara realises she cannot change an entire culture and so they depart in the TARDIS while outside Tlotoxl, now very much in control, sacrifices the Perfect Victim thinking it will appease the gods and so end the naturally occurring eclipse.