This story begins in London around 1885, when two children, Digory Kirke and Polly Plummer, meet. One day, while exploring the attic shared by all the adjoining houses in their terrace, they take the wrong door and surprise Digory's Uncle Andrew in his study. Uncle Andrew, a bumbling yet malevolent self-taught magician, tricks Polly into touching a magic ring (yellow ring), which causes her to disappear. Andrew then blackmails Digory into following her by using another ring, promising him that he will be able to bring Polly back with a different type of magic ring (green ring). The rings transport the children into a wood with many pools of water. Initially, the pools appear to be just shallow puddles but, in a parallel to the attic common to the houses on their street, the children discover that when the correct ring is worn, jumping into different pools will transport the ring's wearer to different universes. Digory convinces Polly to come and explore through some of the other pools with him, expecting his uncle to confiscate the rings upon their return.
After having marked the pool which leads back to Earth, the children enter another pool, and find that they have arrived in the midst of an enormous crumbling palace in the ruins of the ancient capital city of that world, called Charn. The children discover a hall filled with images of all former rulers of Charn, in chronological order. The first faces are fair and wise, but as they progress down the hall, the faces become prouder and crueler. There are still several empty rows, implying a premature end to the kingdom. There they find a bell, as well as a sign which at once dares one to ring the bell and also warns not to ring it. Digory falls for the taunt and rings the bell, against Polly's wishes. It awakens the last of the statues, that of the evil Queen Jadis.
The Queen tells them how she and her sister had waged the final and ruinous war of that world. After many bloody years her own defeat seemed certain, and in order to prevail she had spoken the Deplorable Word. This curse destroyed all life on Charn but that of Jadis, and even she would sit dormant in the Great Hall until someone came to ring the bell. The children, upon learning of Jadis's evil, try to escape back to the Wood but Jadis is able to travel back with them by grabbing hold of Polly's hair, and from the Wood Between the Worlds, to London. Digory and Polly are finally successful in retrieving Jadis, but they bring along not only Jadis, but also Uncle Andrew, a cab driver named Frank, and his horse, named Strawberry.
Digory draws the whole group into the nearest pool, thinking it leads to Charn. This pool, however, leads to a world which appears to be completely dark and empty. Jadis quickly recognises it as a world that has yet to be made. Soon, however, they hear singing which seems to cause the stars to begin to shine and the sun to rise. The visitors can now see the singer for what he is, Aslan, the great Lion, and they continue to watch as he breathes life into the world so that animals, plants, and the world itself are created from nothing. Jadis attacks Aslan with an iron bar she had ripped off a lamp post in London, but as this fails to even attract his attention, she flees, while the iron bar grows into a lamp post in the young Narnian soil. Aslan selects some animals to become intelligent talking beasts, giving them authority over the dumb beasts.
Aslan next sends Digory on a journey to get the apple of youth to protect Narnia, and to atone for bringing the evil witch Jadis into the new world of Narnia. Polly, Digory, and the horse from our world (turned by Aslan into a winged horse and renamed Fledge) fly to a far-away mountain to get the apple from a walled garden. Right as Digory takes an apple and prepares to leave, he sees Jadis, who arrived before him. She tempts Digory to either eat the apple and gain eternal youth, or else secretly transport himself back to London and use it to cure his dying mother. Jadis herself has eaten an apple, thus becoming immortal and proving the power of the fruit. Although tormented by the temptation to steal an apple to save his mother's life, Digory believes that his mother herself would tell him not to steal. He keeps his promise to Aslan and travels back to Narnia to give him the apple.
Aslan tells Digory that he has done well and instructs him to plant the apple in the ground. He then holds a ceremony to crown the king and queen of Narnia (Frank the cab driver and his wife, Helen, whom Aslan magically transported to Narnia). Meanwhile, a new tree grows from the apple Digory planted. Aslan explains that this tree will protect Narnia from the Witch: since she stole an apple from the original tree in a selfish way, its fruit is now abhorrent to her, and Narnia will thus enjoy an innocent Eden-like period. Aslan tells Digory that a stolen apple would have cured his mother, but that the day would have come later when she would have rather died in her illness. Aslan then gives Digory an apple from the tree of protection to take to his mother to save her, and sends the children and Uncle Andrew back to the Wood between the Worlds, whence they return to London. Digory gives the apple to his mother, who is healed, and buries the apple core in his back yard. He also buries the magic rings, which Aslan has instructed him to safeguard to prevent future misuse.
The apple core grows into a tree, and years later it is blown down in a storm. Digory can't bear to have the tree cut up into firewood so he has it made into a wardrobe, linking the end of the narrative to the next story chronologically in the series, The Lion, the Witch and the Wardrobe. The "old professor" in that story is Digory, where he lives in an old country house which he had inherited from his father, who in turn had inherited it from his great-uncle, just after the retirement from his services in India, as described at the end of the book.
