Ten-year-old Jesse "Jess" Aarons has trained all summer to be the fastest runner in his rural school. Secretly, he wants to be an artist, but his father disapproves. He has a crush on the school music teacher, Miss Edmunds, the only person who encourages him to draw.
Jess's new neighbor, Leslie Burke, is a wealthy tomboy from Arlington, Virginia. At recess on the first day of school, Leslie outruns everyone. The other students mock her for being a teacher's pet and for not owning a television. When Jess defends Leslie from Janice Avery, a seventh-grade bully, they become friends. They play by a dry creek behind Leslie's house. They pretend they are the king and queen of a hidden magical kingdom, Terabithia, that can be entered only by swinging over the creek bed on an old rope.
When Jess's six-year-old sister May Belle brings Twinkies to school for lunch, Janice Avery steals them. At Terabithia, Jess and Leslie forge a love letter to Janice from a boy she likes. The letter asks for a date, and Janice is humiliated when he does not show up. Months later, Leslie hears Janice crying in the bathroom. Jess convinces Leslie to help Janice. Janice tells Leslie that she is abusively beaten by her father, and her so-called friends have just gossiped about it to the entire seventh grade. Leslie comforts Janice by telling her that everyone will forget about it in a week. That night, May Belle tells Jess that she followed him and Leslie to the creek. He makes her swear never to follow them again nor to tell their mother.
On Easter, Leslie goes to church with Jess's family. While she calls the story of Jesus "beautiful," she doubts it. This upsets May Belle, who believes God will damn Leslie to hell when she dies. That week, rain turns the dry creek bed into a rushing river. By Wednesday evening, Jess is too scared to swing over the river, while Leslie remains unafraid. On Thursday morning, Miss Edmunds calls Jess and invites him to Washington to visit the Smithsonian Museum. When he returns home, Leslie is dead: The rope broke as she swung over the river, and she drowned.
Jess asks his father whether Leslie is in hell, and his father assures him she isn't. Using a large branch, Jess crosses the stream to Terabithia, where he makes a funeral wreath for Leslie. May Belle, who has followed him, makes it halfway across the branch before becoming too scared to continue. Jess guides her back to land.
Jess's teacher, Mrs. Myers, tells him that when her husband died, people tried to make her forget, but she didn't want to. Using scrap lumber left behind by the Burkes, Jess builds a bridge across the dry creek bed. He puts flowers in May Belle's hair, leads her across the bridge, and begins to play Terabithia, with May Belle as the new queen and Jess as the king.
