The arrival of the TARDIS in 15th century Mexico leads the crew to the doomed Aztec people, a mixture of high culture and brutal savagery. Matters are further complicated when Barbara is mistaken for a god andthe Doctor becomes engaged to be married.

The Doctor, Ian, Barbara and Susan arrive in the TARDIS on board aspaceship. Their initial concern is for the ship's human crew, who are suffering from telepathic interference from the Sensorites, but Susan communicates with the Sensorites and finds the aliens fear an attack by the humans and are just defending themselves. Travelling to the Sense Sphere (the Sensorites' planet) the Doctor seeks to cure an illness to which the Sensorites and Ian have succumbed, but finds it has been caused by deliberate poisoning. The political manoeuvring of the Sensorite City Administrator poses another threat to the TARDIS crew as he seeks to discredit and implicate them.