I am re-watching all of the old New Who in preparation for the 50th anniversary this November, and this two parter always gets me every time I watch it. Epic themes, the Doctor being challenged by a clever and great power. The tension is ramped up in so many ways and evil is fascinating and horrible.
The fact of evil, the Satan figure, being a common cross-cultural idea, and that legends live behind myths when people forget to remember the truths they once knew. The Doctor offers humanity companionship and creativity, and togetherness, that we can find in each other, the strength of endure the vagaries of the world.
The fatalistic humor of the science crew in the face of utter annihilation, remaining alive in an impossible situation through the agency of the unknown. It forces the questions, which all that we can explain, how much more are those things that we have not yet evolved the language to easily share the concept with one another?