The Doctor (William Hartnell), Ian (William Russell), Barbara (Jacqueline Hill) and Susan (Carol Ann Ford) find themselves stranded in Aztec times when the TARDIS is trapped in a tomb. Proclaimed a goddess by the Aztec priests, Barbara attempts to change the course of history by forbidding the human sacrifices which are part of the Aztecs' religion. It is up to the Doctor to find a way back into the tomb and recover the TARDIS, before the priest Tlotoxl (John Ringham) unmasks Barbara.

The Master (Roger Delgado) has stolen some top secret information giving details of one of the most destructive weapons in the known universe, and it is up to the Doctor (Jon Pertwee) to get that information back. The Doctor's quest takes him to the planet Uxarieus, where he becomes caught up in a dispute between the human settlers and the Interplanetary Mining Corporation. Meanwhile, the Master remains at large, and with murderous robots and rampant lizards to contend with, the Doctor's search for the all-important doomsday machine become evermore difficult.