The Descent into the darkness of the Underworld and the Ascent into the Light from the depths of Hell:
Okay, yes, there's the arrival of the TARDIS in the bowels of the Earth and a 'race to the top' narrative structure where they eventually get to Level 1 and the sunlight for the showdown scene, which is similar to the stories of Heracles, Odysseus, Orpheus (Oh Orpheus), Aeneas, and Dante.
But it's the metaphoric dimensions of that structure that fascinate me more...
So, neither the Doctor nor the Dalek are entirely good/evil in this one. it isn't a nice, clear, safe binary where the Doctor is the Hero and the Dalek is the Villain.
Instead, they are presented sharing conjoined aspects of the same soul, except they're moving in opposite directions to one another. Due to Rose's touch, the Dalek's 'humanity', for want of a better term, ultimately comes to be redeemed but at the same time it's the presence of the Dalek - of the darkness - that sends the Doctor into a negative spiral of descent and he becomes something he despises in the form of essentially 'a good Dalek' (to the point, by the way, where he is willing to sacrifice Rose!).
I think in that representation of varying shades of grey we get a really quite a poetic, Jungian take on archetypes of the Self and the Shadow which doesn't allow for a neat, simple, simplistic Black and White take on 'goodies' and 'baddies'. There can come to sprout redemption in the most evilllest of souls and there is corruption even in the hearts of the Doctor.
(I think, also, that this has particular resonance when we step back and see that both the Daleks and the Doctor are really extrapolations of opposite ends of a wide spectrum that is humanity as evidenced by its heroes, and history, and villains.)
This is where New Who arrives, and where maybe it sets itself apart from the largely traditional stories of Classic Who. (It's surpassed even postmodernism - it's now poststructuralist.)