With the episode “Planet of Fire,” Doctor Who revisits some classic tropes of religious skepticism. On a volcanic world inhabited by a people many of whom are devoutly religious, faith comes to the fore very early. But by the end of the episode, the central deity of their belief system, Logar, turns out to have been a person from another world wearing a heat-resistant thermal suit. Religion is thus viewed as the attempt of primitive people to make sense of their encounter with extraterrestrials – in this case, inventing legends and myths to make sense of what they saw, which was in fact a vulcanologist from the planet Trion.
The Master also plays a key role. Having been accidentally shrunk, he is now seeking to be “born again” (that phrase is actually used in the episode) with the help of numismaton gas (which sounds like it ought to stem linguistically from pneuma, the Greek word for spirit, associated in obvious ways with ideas of rebirth).
This episode witnessed the introduction of Peri, the Doctor’s first American companion. It also saw the departure of Turlough, who was in fact an exile from Trion, and since their civil war had ended, he was now free to return. Kamelion too (largely ignored in the previous several episodes due to the robot prop being glitchy) met his end.