The narrative, as recounted by by protagonist Aristotle (Ari), begins in summer. Fifteen year old Ari, in spite of being unable to swim, visits a nearby pool in his hometown of El Paso, Texas. There he meets another fifteen year old, the talkative Dante, who teaches Ari to swim and, along the way, becomes what Ari describes as his first real friend.

As the friendship between Ari and Dante deepens, they become friends with each other’s parents; discover shared interests and different attitudes; and begin to awaken to feelings for each other that go beyond friendship, an intimacy with which Dante is far more comfortable and open than Ari. At one point, an encounter with a skidding car results in Ari saving Dante’s life, but suffering two broken legs in the process. Dante and his parents express deep gratitude but Ari, who tends to be uncomfortable with emotion (both his and other people’s) brushes off their thanks. Meanwhile, as an incentive towards recovery, Ari’s father buys him a truck and, after the casts on his legs are removed, Ari (after celebrating his sixteenth birthday) learns to drive, and revels in his new freedom. Around this time, he also discovers a sealed envelope with the name of his brother (who is in prison and who is never spoken about) on it.

That fall, Dante accompanies his parents on a move to Chicago: his father, a university professor, is filling a short term contract. In his narration, Ari describes his efforts to fill his time with schoolwork, with fantasies about the beautiful Ileana, with taking care of his truck, and with taking care of his new dog, a stray mutt that attaches herself to him while he is strolling past Dante’s home. Ari also answers some of Dante’s letters, but not all: Dante is just as talkative on paper as he is in person, his letters chatting about his experiments with kissing, with drugs/alcohol/partying, and with art. Dante’s openness and curiosity, particularly when it comes to physical and/or sexual matters, tend to make Ari uncomfortable, and he takes refuge in brooding more deeply about his brother.

The second half of Dante’s year of absence passes quickly. As summer returns, so does Dante, and he and Ari resume their friendship, although not quite as easily as before: Dante is becoming increasingly comfortable with his emerging same-sex attractions, an aspect of his friend’s character that makes Ari uneasy. His affection and protectiveness towards Dante overcome that unease, however, when Dante is attacked after being found kissing another boy. While Dante recovers in hospital, a blindly furious Ari takes revenge on the attacker, beating him up in turn.

Ari escapes serious consequences for his actions, but is forced by his parents to face some uncomfortable truths: his older brother went to prison for giving in to the same sort of violent feelings as Ari did in avenging Dante. Ari tearfully assures them that he is not like his brother, and will do everything in his power to avoid the same fate. Ari’s parents also reveal their belief that he has long been in love with Dante and assure him of their support and love. Shortly afterwards, Ari comes to accept that they’re correct, and confesses his feelings to Dante. The two kiss, and spend a romantic evening lying in the back of Ari’s truck, looking up into the night sky, now knowing a few more of the secrets of the universe.