The TARDIS arrives on Earth landing in 16th century Aztec society. Barbara goes off exploring - despite the Doctor's orders not to wander off - and is taken away by the high priest Autloc who believes she is the reincarnation of the goddess Yetaxa. They believe it is a sign that the rains - which are late - will now come. The high priest of sacrifice Tlotoxl recruits Ian to lead their army but he finds he has a rival in Ixta. Ian is less than comfortable with his role when he learns that they are to deliver the human sacrifice to the high priest at the rain ceremony. The Doctor emphasizes that they must not interfere with Aztec rituals, something that does not sit well with Barbara.

Barbara tries to change the course of history - and Ian fights for his life.

While the Doctor tries to find a way back to the TARDIS, Tlotoxl continues to plot Barbara's downfall and sees an opening when Susan questions Aztec custom.

Tlotoxl's machinations result in both Susan and Ian being arrested. With the TARDIS once again trapped inside the tomb, the travellers' only hope for escape lies with Autloc and Cameca.