What is the major conflict in The Magician's Nephew?

In all of the books in the series The Chronicles
of Narnia, the over-riding conflict is good versus evil, most of which
involves Aslan versus the White Witch. The individual novels all have different stories,
however, where there are other characters whose conflicts against  evil are more in the
forefront. Aslan is always present at some point in all of the novels, so this is always
the battle that supersedes the other ones.

In The
Magician's Nephew, readers find out how humans first entered the kingdom of
Narnia - through a magic apple planted in Digory's backyard that grew up into a tree,
from which the wardrobe was constructed  that allowed the Pevensie children to enter
Narnia while visiting the old professor, who is Digory grown up. Digory, however, is the
magician's nephew because his uncle Andrew is the magician. In this story, the evil
queen Jadis is in conflict with Digory and Polly, who mistakenly bring her to London. As
with all the other stories, Aslan must show up to rescue the humans, however, so there
is always the main conflict of good vs evil despite the different story
lines.