In the summer of 1987, 15-year-old Aristotle Mendoza meets a boy named Dante Quintana at the local pool. The boys bond over their classical names and eventually become inseparable. Dante teaches Ari about literature and poetry, while Ari is fascinated by Dante's swimming ability and sincerity.
Dante tells Ari that he and his family are moving to Chicago for the next school term because his father was offered a temporary professorship at the University of Chicago. That same day, the two boys see a bird lying injured in the road. When Dante goes into the road to check on the bird, a car speeds around the corner. Ari dives into the street and pushes Dante out of its path, saving his life. While Dante leaves almost unscathed, Ari is hurt very badly. Following the accident, the Quintanas and the Mendozas grow closer. Both boys' mothers talk more frequently and share ideas about their sons.
Before Dante leaves for Chicago, he tells Ari that the two things he loves most in the world are swimming and Ari. However, Ari says that he should not tell him those things, even if they are true. The two boys promise each other that they will still be friends when Dante returns in the summer.
Over the next year, Dante sends Ari several letters detailing his life in Chicago and struggling with his sexuality. Ari learns to drive, falls in love with a girl from school, and searches for answers to his questions about his brother Bernardo, who is in prison for reasons no one in his family will discuss.
The next summer, Dante convinces Ari to kiss him as an experiment. It becomes increasingly clear that Dante is in love with Ari, who appears not to reciprocate Dante's feelings for him.
Ari's father announces that his Aunt Ophelia has had a fatal stroke. At the funeral, Ari realizes that none of his extended family is there. He is told that they disapproved of Aunt Ophelia having lived with another woman for many years. After the funeral, Ari's mother explains that Ari's brother, Bernardo, was arrested for the murder of a prostitute he hired when he was 15 years old. When Bernardo found out the prostitute was transgender, he killed her with his bare fists.
When Ari returns home, Mr. Quintana tells him that Dante is in the hospital. He was jumped by several young men who had seen him kissing his boyfriend Daniel in an alley. Ari tracks down Julian, one of the boys who attacked Dante, at the body shop where he works and starts a fight with him. Mr. Quintana asks if Ari knows why Dante was jumped. Ari tells him that Dante is gay and was kissing another boy. Mr. Quintana admits that he'd guessed the truth because of the way Dante looks at Ari, while Mrs. Quintana tells Ari she thinks Dante is in love with him and that Daniel is just a stand-in for Ari.
Ari's mother eventually calls a family meeting, where Ari finally accepts that he is as much in love with Dante as Dante is with him. That night, the two families go bowling together. After bowling, Dante and Ari go out into the desert, where Ari kisses Dante, fully accepting his love for him. Now free of his fears, Ari is left wondering: "How could I have ever been ashamed of loving Dante Quintana?"
