Peter, Susan, Edmund, and Lucy, four siblings in World War II-era England, are sent away from London to live in the countryside, where they will be safe from air raids and bombings. While they are staying in a large, historic country house owned by an old professor, Lucy discovers that a certain wardrobe in a spare room is a gateway to a magical land called Narnia. In Narnia, Lucy meets and becomes friends with a Faun named Mr. Tumnus, who tells her that Narnia is ruled by an evil White Witch who oppresses the people and magically creates an everlasting winter. Lucy returns home and tells her brothers and sister about her experience, but they don't believe her because the wardrobe now has a normal wooden back.
One rainy day, Lucy returns to Narnia through the wardrobe and Edmund follows her. While Lucy goes to visit Mr. Tumnus, Edmund is left alone and discovered by the White Witch as she travels through the countryside on her sledge. The Witch coaxes Edmund into telling her about his family and is disturbed to learn that there are four of them, two boys and two girls. She bewitches Edmund with an enchanted version of his favorite candy, Turkish Delight, and convinces him to bring his brother and sisters to her. After the Witch departs, Lucy discovers that Edmund has entered Narnia. Returning home again, she is sure he will back up her story, but instead he lies, telling Peter and Susan that Lucy's Narnia is make-believe.
Some time later, all four children are forced to hide in the wardrobe to escape from the housekeeper, Mrs. Macready, and a group of sightseers touring the country house. All four children find themselves in Narnia. Lucy takes them to visit her friend Mr. Tumnus, but they find that he has been arrested by the White Witch's secret police. While they are deciding what to do, a robin leads them to a talking Beaver, who introduces himself as a friend of Mr. Tumnus. Mr. Beaver takes the children home to his dam, where he introduces them to his wife Mrs. Beaver. The Beavers feed the children a solid meal and explain about the prophecies of Narnia: when four human beings, two male and two female, sit in the four thrones at Cair Paravel, a castle on the country's eastern coast, then the White Witch will be destroyed. The Beavers also tell the children about a lion named Aslan, the Lord of the Wood, who has returned to Narnia after a long absence. Aslan has the power to end the winter created by the Witch.
While the Beavers are explaining Narnian history and prophecy to the children, Edmund sneaks away and goes to the White Witch's house, where he discloses all their plans. He is horrified when the Witch treats him coldly and reveals that she intends to capture and murder his siblings. Taking Edmund as a hostage, she attempts to intercept the Beavers and the children by traveling quickly on her sledge. However, in a few hours, the endless winter dissolves into a beautiful spring, and the Witch is forced to march across country with Edmund and one servant, a Dwarf.
Meanwhile, Peter, Susan, Lucy, and the Beavers have escaped, taking only a little food with them. They head toward the Stone Table, an ancient monument, where Aslan is rumored to have set up camp. During their journey, they meet Father Christmas, who gives them weapons to use in the battle he anticipates between Aslan's forces of good and the White Witch's forces of evil. When they arrive at the Stone Table, they are awed by Aslan's presence, but Aslan is saddened by the news of Edmund's betrayal. Aslan speaks to Peter of Cair Paravel, but they are interrupted by Fenris Ulf, a fierce wolf in the service of the Witch. Peter slays Fenris and is knighted by Aslan.
Nearby, the Witch has decided to murder Edmund to prevent fulfillment of the prophecy. Just before she strikes, Edmund is rescued by some of Aslan's people. The Witch goes to see Aslan under a flag of truce and demands her right to Edmund's blood, citing the Deep Magic from the Dawn of Time, which gives her control over all traitors. Aslan comes to a private agreement with the Witch and she renounces her claim on Edmund. Depressed and subdued after this meeting, Aslan orders his people to move their camp away from the Stone Table.
At night, Aslan leaves camp alone and goes back to the Stone Table. Lucy and Susan follow him – feeling, but not understanding, his sorrow. As he gets close to the site, he insists that they remain behind, hidden. When Aslan walks into the clearing by the Stone Table, the White Witch and all the evil creatures she has gathered are there to meet him. They bind and torment him, but he bears their cruelty with patience. In the end, the Witch murders Aslan with an enormous stone knife. Then all the evil creatures rush away to do battle with Peter, Edmund, and Aslan's other followers.
Susan and Lucy weep over Aslan's dead body. With the help of some friendly mice, they remove the cords and muzzle that bind Aslan. All through the night, they mourn. At sunrise, the Stone Table breaks in half with a loud noise and Aslan is miraculously resurrected. He explains to the astonished girls that there is a Deeper Magic from Before the Dawn of Time: because he was an innocent, willing victim and was sacrificed in place of a traitor, the Stone Table broke and Death worked backwards. The girls frolic with Aslan, who is feeling lively. Then they all rush over to the Witch's house and free her captives. Aslan is able to reverse the effects of the Witch's magic wand, turning many people and creatures from stone statues back into themselves.
With this new band of followers, Aslan and the girls return to the others, where they find Peter and Edmund fighting a losing battle against the Witch. The freed captives quickly turn the tide of battle as they join in on the side of Good, and Aslan kills the Witch. Lucy uses her Christmas present, a magic cordial, to heal the wounded, and Aslan cures those who were turned to stone by the Witch. Everyone goes to Cair Paravel, the castle on the eastern sea, where Peter, Susan, Edmund, and Lucy are crowned as kings and queens. They reign for many years and Narnia prospers, although Aslan comes and goes.
One day, while hunting a stag that grants wishes, King Peter, Queen Susan, King Edmund, and Queen Lucy find a lamppost in the woods. They begin to remember their lives in England, and as they go further into the woods, they find themselves back in the wardrobe, and then back in the spare room in the country house. No time has passed in England since they first entered Narnia together, and they are children again.