In 1940, four siblings – Peter, Susan, Edmund, and Lucy Pevensie – are among many children evacuated from London during World War II to escape the Blitz. They are sent to the countryside to live with professor Digory Kirke.
While the four children explore the house, Lucy climbs into a wardrobe and discovers that inside it is a magical forest in a land called Narnia. At a lamppost in the midst of the forest she meets Mr. Tumnus, whom she discovers is a faun. She accepts his offer to have tea in his home. After tea, however, the faun sadly confesses to Lucy that he invited her not out of hospitality, but with the intention of betraying her to the White Witch. He explains that the witch has ruled Narnia for years and during her reign has used evil magic to make it always the same season: winter. As Tumnus explains, it is "always Winter, but never Christmas." The witch has ordered all Narnians to report or capture any Sons of Adam or Daughters of Eve (which is how Narnians refer to human beings). Mr Tumnus' intention had been to obey that order and hand Lucy over, but now that he has met a real human, Mr Tumnus feels incapable of obeying the witch's orders, so he repents of his original intention and escorts Lucy back to the lamppost.
When Lucy returns through the wardrobe, she discovers that only a few seconds have gone by in normal time during her absence. Hence, her siblings do not believe her story about there being another world inside the wardrobe. She later returns to Narnia during a game of hide-and-seek with her siblings. Her older brother Edmund, who had been particularly spiteful in his refusal to believe her story, enters the wardrobe after her and gets into Narnia as well. While searching for Lucy, he meets a lady who introduces herself as the Queen of Narnia. She is very interested in Edmund and questions him about his family. When she learns he has two sisters and a brother, she enchants him with magical Turkish delight sweets, whose enchantment is like a drug that leaves Edmund longing for more. The witch refuses to give him any more enchanted sweets until he goes back into his own world and brings his three siblings back with him. She says he must bring them with him to her castle. She points out where her castle lies, so that he will know how to get there when he returns. She emphasises to him the importance of making sure he brings his brother and sisters with him when he comes, and tells him that when he comes back she will make him into her prince and heir.
Lucy discovers Edmund by the lamppost on her way back from visiting Mr Tumnus and is delighted to find that he has also gotten into Narnia. They return together through the wardrobe. In conversation with Lucy, Edmund realizes that the lady he met was in fact Jadis, the White Witch, but the effect of the enchantment still lingers on him, and he does not tell anyone he has met her. He also lies to Peter and Susan, denying Lucy's claim that he too had entered Narnia. Unable to decide whether or not Lucy is lying or even delirious, Peter and Susan take the matter to the Professor. Much to their surprise, the Professor appears to take Lucy's side and gently chides them for their cynicism.
Soon afterward, all four children enter Narnia together while hiding in the wardrobe after an encounter with the professor's housekeeper, Mrs Macready. Peter is angry with Edmund when he learns that Edmund had been to Narnia after all. Lucy guides them to Tumnus' cave, but finds it ransacked, with a notice from Jadis' police about his arrest for high treason due to fraternising with humans. Lucy realises she is the human in question.
They are spotted by Mr. Beaver, who guides them to his house. Mr. and Mrs. Beaver (who are able to talk) tell them of a prophecy that Jadis' power will fail when two Sons of Adam and two Daughters of Eve fill the four thrones at Cair Paravel. The Beavers tell them of Aslan, the great lion who is the rightful King of Narnia. He has been absent for many years but is now "on the move again".
Having heard enough, Edmund sneaks away to Jadis' castle, which is filled with statues of Narnians she has turned to stone. Jadis is furious with Edmund for coming alone and angrier still when he informs her what he has heard the Beavers say: Aslan is back in Narnia.
Meanwhile, back at the cottage, the other three children are trying to work out where their missing brother has gone. Mr. Beaver realises Edmund has gone to the White Witch. He tells the three siblings that he knew from the look on Edmund's face that he had been in the witch's company. Knowing the White Witch will soon come to get them as soon as Edmund tells her where the other three children are, the Beavers abandon their home and set out on a trek through the snow to lead the three children to Aslan. During their journey the snow begins to melt as Jadis' spell over Narnia starts to break due to Aslan's arrival. Winter ends, and it is finally Christmas, so Father Christmas appears during their journey, with presents for the three children and the beavers. Lucy is given a dagger and a special cordial that can heal the sick; Susan is given a bow and arrows, and a special horn that will always bring help when blown; and Peter is given a sword.
Aslan welcomes the children and the Beavers to his camp at the Stone Table, and Peter is soon required to make use of his new sword by killing Maugrim, the chief wolf of Jadis' Secret Police, who tries to kill Susan. Peter is victorious, and Aslan makes him a knight. Aslan's troops run after a second wolf that runs away, and it leads them to the enemy camp, where they rescue Edmund just as Jadis is about to kill him.
Edmund is brought back to Aslan's camp to join the others, and Jadis approaches in truce to parley with Aslan, insisting that, according to "deep magic from the dawn of time", she has the right to execute Edmund for treason. Aslan speaks with her privately and persuades her to renounce her claim. That evening, Aslan secretly leaves the camp, but Lucy and Susan follow him. It appears that Aslan has bargained his own life for Edmund's, for the girls witness Jadis tie Aslan to the Stone Table and kill him with a knife. The next morning the enemies are gone, and only Susan and Lucy remain weeping over Aslan's body, when some mice come and bite through the ropes the enemy had used to tie him. Then the Stone Table is broken and Aslan is restored to life, telling Lucy and Susan that "deeper magic from before the dawn of time" (which Jadis did not know about) will resurrect an innocent killed in place of a traitor.
Aslan allows Lucy and Susan to ride on his back as he hurries to Jadis' castle. There he breathes upon the statues, restoring them to life. Meanwhile Peter and Edmund are leading the Narnian army against Jadis' army, and many of the loyal Narnians are being turned to stone. Edmund attacks Jadis and destroys her wand. Jadis, no longer able to turn soldiers to stone, fights with the stone knife instead. Edmund is seriously wounded. Aslan arrives with the former statues as reinforcements. The Narnians rout Jadis' army, and Aslan kills Jadis. Aslan walks around the battlefield breathing on those who have been turned to stone to bring them back to life, and Lucy uses her magic cordial to revive first Edmund and then all the others who have been wounded.
After the battle, the Pevensie children are crowned kings and queens of Narnia at the castle Cair Paravel. They are given the titles of King Peter the Magnificent, Queen Susan the Gentle, King Edmund the Just, and Queen Lucy the Valiant. After the coronation Aslan slips away and disappears. Lucy is sad to see him go but is reminded that he is "not a tame lion" and will come and go as he pleases.
Fifteen years later, the siblings are hunting for a white stag when they find the lamppost in the forest. Beyond it, the branches become coats. They come back through the wardrobe in the Professor's house and are suddenly children again. This is because, during their whole stay in Narnia, almost no time had gone by in the real world, despite so many years' having gone by in Narnia time.