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All of the books in the series The Chronicles of Narnia are allegories regarding the triumph of good over evil. The individual novels all have different storyliness but they have something to do with the imaginary land of Narnia and/or Narnia's great hero, Aslan. Aslan is always present at some point in all of the novels, and he is a Christ figure.
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, Digory and Polly bring the evil queen Jadis to London by mistake. Aslan must show up to rescue the humans in this story, just as with the others. Digory has a chance to steal a magic apple to heal his mother from her illness, but he chooses not to do this knowing it is wrong and knowing that his mother would not approve. So there is the additional theme of choosing the right thing, even though it is difficult. Aslan rewards Digory for his honesty and gives him an apple anyway, which he uses to heal his mother and later plants in the backyard.