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.