Jessie Oliver Aarons, Jr., “Jess”, wakes up early each day to practice running; he is determined to be the fastest runner in all the 3rd, 4th, and 5th grades. One day, a U-Haul is parked next door. Jess shrugs it off; he doesn’t realize that he is dismissing one of the most important moments of his life. Jess goes to his room and begins to draw. Nobody at home or school supports his drawing hobby, except Miss Edmunds, the music teacher.
The next morning Jess doesn’t feel like running, but he pushes himself. In the middle of his run, he is stopped by a voice. At first he cannot tell whether she is a boy or girl, but eventually figures out it belongs to a girl, Leslie Burke. He avoids talking to her and goes home to do his chores.
When school starts the following week, Leslie is in Jess’s class. Jess continues to avoid her and waits impatiently for recess, eager to be the fastest boy and beat Gary Fulcher. At recess, Leslie joins the boys to race too. Gary tells her that she can’t race, but for some reason, Jess insists that she does. Leslie beats Jess in the first heat and beats the rest of the boys in the finals; everyone is stunned. For the next few days the boys continue to race, and Leslie wins every time. By Friday, nobody wants to race anymore.
Leslie is not like other girls: she doesn’t wear the same kinds of clothes, she has different hobbies, and she doesn’t own a television, even though her parents are rich. After some kids give Leslie a hard time, Jess sticks up for her and they become closer friends. They decide to build their own world, Terabithia, in the woods near their homes; here they are the King and Queen of their land and go on many adventures.
Janice Avery picks on anyone smaller than her, including Jess’s younger sister, May Belle. May Belle insists that Jess beat Janice up, but Leslie convinces her that it is a bad idea. Instead, Leslie and Jess come up with another plan: Jess forges a letter, pretending to be Janice’s crush, Willard Hughes, and claiming to be in love with her. The two sneak into Janice’s classroom and place the note in her desk. Janice waits after school for Willard to walk her home as promised in the note, but when he doesn’t show, she is beyond angry. Jess feels bad for what they had done, but Leslie believes Janice deserved it.
At school, Janice Avery begins crying in the bathroom and Leslie tells Jess about it. He convinces her to go back into the bathroom and help Janice. Janice is upset that her best friends told the seventh grade about how her father beats her. Leslie is happy to have helped Janice and even calls her half of a friend. Jess is thrilled that Leslie calls him one whole friend, because he knows that she is his only true friend too.
Jess’s dad gets laid off from his job in Washington, and his sisters are upset about not getting new clothes for church on Easter. However, they finagle a way to get new clothes and Jess receives nothing. He asks permission to bring Leslie to church with them because she has never been to church before. His mother agrees and Leslie wears a dress and nice shoes for the first time. She believes that the story of Jesus is beautiful, but May Belle insists that it is scary to have holes nailed into your hands. She tells Leslie that those who do not believe in God will go to Hell. Leslie doesn’t believe in God, and Jess and May Belle feel uncomfortable about what will happen to her.
In the spring, it rains for several days, and after many days of not visiting Terabithia, Jess and Leslie decide to go to their “castle”. The water in the creek has risen high, and Jess is very afraid of crossing it, but continues because Leslie is fearless. Jess is cold and miserable and is thankful that Leslie suggests going home to watch television. That night, Jess awakens thinking about crossing the creek and decides to tell Leslie that he doesn’t want to go there anymore; he hates the feeling of being afraid and he won’t cross the creek again.
Jess gets a phone call from Miss Edmunds. She asks if he will be able to go with her to Washington to visit the Smithsonian. He spends the day with her exploring the art and enjoying Miss Edmund’s company. Returning home, he is giddy, until he realizes that something is wrong his family are not eating dinner, or watching television. His sister blurts out that Leslie has died, and they thought Jess was dead too.
Jess runs down the road until his father catches up with him in the pickup truck and puts him to bed. In the morning Jess remembers having a bad dream, but can’t remember the details. He replays his day with Miss Edmunds, but has an inexplicable feeling of dread. Jess eats pancakes for breakfast, not seeming to be affected by Leslie’s death. Jess’ dad suggests they go pay the family respects and they walk over to the Perkins place. Jess is confused and almost annoyed that everyone is crying. However, after hearing about Leslie’s cremated body being sent to Pennsylvania, he runs from the house in tears, punches May Belle in the face, takes the paint set Leslie gave him, and throws it in the creek. His father sits and consoles him, easing his worries about Leslie going to Hell.
Jess uses a branch as a bridge to cross the creek to visit Terabithia and pay his respects to Leslie in some way. He decides to make a wreath for her. He hears a scream coming from the creek. May Belle had followed him, but couldn’t make it across the bridge. Although his heart was pounding, he calmly helps her back to the house. Thinking of Leslie going into the bathroom after Janice Avery, she tells May Belle that everyone, even Leslie, gets scared sometimes.
At school Jess is miserable, hearing all the whispers from other kids and having Leslie’s desk already removed from the classroom. Mrs. Myers pulls Jess into the hallway, and with tears in her eyes, tells him how sorry she is. Jess sees Mrs. Myers in a new light and is appreciative of her kind words. Jess realizes how Leslie has helped him change from a weird kid to a King in Terabithia and how his eyes have opened to the beauty of the world.
Leslie’s parents move back to Pennsylvania. They give Jess all of Leslie’s books and her paint set. Jess asks to have some lumber and he builds a bridge across the creek. May Belle follows him and asks what he is doing. He tells her it is a secret until it is done. As she crosses the bridge Jess imagines that all the people of Terabithia are standing on tiptoe to watch the new queen, May Belle, approaching.