In the setup to the book, we meet Jess, learn about Lark Creek (see "Setting"), and figure out a little bit about his family life. We see that Jess isn't understood by his family, doesn't have any real friends, and doesn't even like school very much. So, he takes all the feelings he has about loneliness or being left out, or wanting something more for himself, and puts them into the goal of training to become the fastest runner in his grade. Being the fastest runner will give him identity, glory, and purpose. It's the most exciting thing on his horizon.
Even though Jess and Leslie are both looking for friends, and they've just become neighbors, Jess doesn't start out thinking Leslie's an appropriate friend choice. After all, she's a girl, and at the beginning of the book his understanding of friendship is kind of limited by gender roles. Yet, even though he barely knows Leslie, he stands up for her when she wants to run in the boys' race. He doesn't even know why he does so, and kind of regrets it when she beats the pants off everybody else. On just her first day at school, she's ruined his dream of being the fastest runner. But her efforts persist and eventually she wins Jess over. By losing the title of fastest runner, Jess gains a surprising new friend.
Jess and Leslie's friendship is strengthened by their two encounters with Janice Avery, the seventh grade bully. First, Janice takes May Belle's Twinkies. Since Jess can't try and beat Janice up for revenge, he and Leslie concoct a plan whereby they write her a fake love letter and embarrass her. They use ingenuity to achieve a better end result. Later, though, when Leslie finds Janice crying in a bathroom, she and Jess find they've grown in compassion and make an attempt to comfort the former bully. Together they learn that even enemies should be treated with dignity.
Jess is thrilled beyond expectation when Miss Edmunds offers to take him out for a special day. He's so taken aback that he forgets about his worry that it wouldn't have been safe to go to Terabithia that day. Throughout his time with Miss Edmunds, he thinks about how Leslie would react and how she would advise him in different situations. Ironically, on his way home he thinks that the day was so great it would be worth any sacrifice. Little does he know that it will cost him something tremendous.
At first Jess can't believe his friend has died – he refuses to process the information. He tries to deny it when people tell him, and then starts acting like nothing happened. He even dreams that her death was another, worse dream, and tries to talk to her and tell her about the magical day he had – a day that will forever be tainted. It's not until he throws Leslie's gift to him away that he begins to realize she's actually gone and is finally able to cry.
Jess is still struggling to process his grief when he admits to his father his worry Leslie will go to hell, and his dad reassures him that their God wouldn't do that. What helps Jess most might be talking to his teacher Mrs. Myers, who reminds him that he has the power of memory and can keep Leslie with him, in a way, by honoring her memory and thinking about her. By observing her grief, he learns about human nature and realizes just how much he learned from Leslie after all.
At the end of the book, Jess returns to Terabithia and has to save his sister's life when May Belle tries to follow him. Using lumber given to him by Leslie's parents, Jess is inspired to build a bridge across to Terabithia, so that both he and May Belle can get there safely. While we're sure he wishes they'd built the bridge months earlier, so Leslie would have been safe, the fact that he builds it after her death means that he still values Terabithia and thinks it's important to keep going there, rather than staying away after the tragedy. Leslie would have wanted it that way.