The TARDIS lands in an elaborate tomb. Barbara instantly recognizes from her studies that they are in the 15th century Aztec Empire. She takes a bracelet from the skeleton.
Susan pushes on a side of the tomb and it opens. Barbara explores the temple but is trapped when the door closes. Autloc, the Priest of Knowledge, confronts her but stops when he sees the bracelet.
Susan, Ian and the Doctor follow Barbara but are also trapped when the door closes behind them. They are greeted by Autloc as the "servants of Yetaxa". The bracelet has convinced him that Barbara is a reincarnated priest. Tlotoxl, the Priest of Sacrifice, boasts to Autloc that their pending human sacrifice will bring the rains, but Autloc believes the rains will come regardless.
The Doctor, Ian and Susan are brought to Barbara, who has been dressed as a priestess. The Doctor urges Barbara to sustain their belief or they will be killed. The High Priests ask Barbara to present herself to the people to persuade them of the gods' good will. She agrees.
Tlotoxl suggests that Ian be allowed to lead their armies in place of Ixta, the current champion. He brings Ian to Ixta, who demonstrates his skill by defeating another warrior in combat. He then boasts he will kill Ian to prove his leadership. But their first duty will be to bring the human sacrifice to Tlotoxl that afternoon.
Autloc brings the Doctor to a garden reserved for the elderly. There, the Doctor meets Cameca, an Aztec woman. She tells him the garden was built by the same man who built the temple. The architect is dead but his son still lives and may be able to help the Doctor.
Ian arrives in warrior dress and tells the Doctor of his role in the sacrifice. The Doctor makes him promise not to interfere.
The Doctor returns to Barbara to give her the same message. But she refuses to listen, convinced that she can change the Aztecs so that they will not be destroyed by Cortez.
The ceremony begins and Barbara is brought out to see a victim splayed on the altar. She is presented to the people. Tlotoxl starts the sacrifice but Barbara tries to stop him. The victim is shamed and jumps to his death from the top of the pyramid. The rains fall immediately after.
Tlotoxl wants Susan punished for crying out during the sacrifice but Barbara refuses. This convinces Tlotoxl that Barbara is not the resurrected Yetaxa. She is a false godess and he will destroy her…