The Doctor’s spacetime ship ‘Tardis’ departs the planet Marinus to arrive atop a sixteenth century Aztec pyramid on Earth. History teacher Barbara tries on a bracelet in the temple’s crypt and is found by the high priest Autloc, who believes she is the reincarnation of their god Yetaxa. The discovery is taken as a sign that the rains – which are late – will now come.
The high priest of sacrifice Tlotoxl recruits Ian to lead their army but the science teacher finds he has a rival in the warrior 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 emphasises that they must not interfere with Aztec rituals, something that does not sit well with Barbara.