Such a sad little book.
Jess is isolated from his family (dad works far away and he's left home with his many sisters and mom), has few friends, and is scorned by his father for his obsession with art and drawing. His only friend, really, is the cow he has to milk twice a day and, sometimes, his young sister.
Then Leslie moves in down the block and the two become fast friends. Leslie is the daughter of hippies and is very much a free spirit. The two misfits spend the better part of a school year hanging around together, and create their own magic kingdom (Terabithia) in the woods near their houses.
They act as king and queen and fight off imaginary invaders and rule Terabithia as benevolent monarchs.
The one day while Jess is on a trip to Washington DC with his hot music teacher, Leslie has an accident and is killed. (Technically, this is not a spoiler, since this information is on the back cover.) Jess's reaction and recovery is the focus of the last chapter of the book. He builds a bridge to cross the creek where Leslie died trying to get to Terabithia. To honor Leslie's life and their friendship, Jess invites his sister to co-rule Terabithia with him.