Jess Aarons trains to be the quickest runner in fifth grade. During the summer, he wakes up early to train. At Jess’ school, Lark Creek Elementary, being a good runner is the best way for a fifth-grade boy to distinguish himself. Jess is an artist and has previously spent most of his free time drawing, but now he wants to be an athlete.
Jess notices a family moving into a house nearby, and the daughter of the new family stops by for a visit. Leslie Burke is Jess’ age, and she tries to befriend him, but Jess resents Leslie when she beats him in a race at school. His dreams of being the best runner in his grade are dashed when he learns that no amount of practice will make him as fast as Leslie. Jess decides to make friends with Leslie anyway, and he learns that she is a thoughtful and interesting person.
Leslie decides that she and Jess need to invent their own secret country where the two of them can escape from the pressures of their difficult lives. They pick a place in the woods behind Leslie’s farmhouse and dub their new kingdom “Terabithia.” Jess and Leslie visit their land together daily. Jess provides Leslie with support and companionship, and Leslie improves Jess’ world by teaching him new words and telling stories.
Jess and Leslie decide to take revenge on an older girl, Janice, who is unkind to them and who steals things from weaker children. Jess and Leslie fool Janice into thinking she has received a love note from the boy she likes. When Janice tells her friends about the love note, they discover that the note is fake, and Janice is humiliated in front of the whole school.
As Christmas nears, Jess worries about what kind of present he will buy for Leslie. Jess gives her a puppy, and Leslie names it “Prince Terrien, guardian of Terabithia.” Leslie gives Jess a watercolor set. By spending part of Christmas vacation with Leslie, Jess feels less worried about the argumentative atmosphere of his own home. Jess gradually grows closer to Leslie’s parents as he helps the Burkes repair and decorate their farmhouse.
At school, Leslie hears Janice crying in the girls’ bathroom. Jess feels sorry for Janice and asks Leslie to see if Janice needs help. After talking to Janice, Leslie discovers that Janice’s father beats her and that the secret has been spread around school. Leslie comforts her, and the two of them become friends.
In the spring, Jess and Leslie continue to play in the forest, but they are surprised by how much the creek has risen from the spring rains. Jess is afraid of crossing it, but he thinks that he will feel braver after he has taken swimming lessons from Leslie, whose favorite hobby is scuba diving.
Jess’ music teacher, Miss Edmunds, asks Jess to go with her for a one-on-one field trip to the Smithsonian museum and the National Gallery in Washington, D.C. When Jess returns from the trip, his family tells him that Leslie has died. After falling and hitting her head, Leslie drowned in the flooded creek behind her house. Jess is overwhelmed by sadness and disbelief. He does not know how to react to Leslie’s mourning family, and he feels angry with Leslie for abandoning him.
Jess goes into the woods to think about the times he spent with Leslie, but his little sister May Belle follows him. May Belle begins to cross the tree bridge over the flooded creek, but she loses her courage halfway across and yells for Jess to help her. Jess brings May Belle to safety on the shore of the creek and discovers that she was trying to join him so that he would not be lonely anymore.
At school, the other students whisper about Jess, but no one speaks to him. A teacher named Mrs. Myers tells Jess that she is sorry for his loss. Mrs. Myers misses Leslie, too, and she says that she and Jess can help each other through the time of loss. Jess understands that Leslie made his life better just by being his friend, and he does not want to forget her. Jess decides to start playing in the land of Terabithia with his sister to show her some of the affection and affirmation that Leslie shared with him.