The events, set in Mexico in 1999, are punctuated by a voice-over, omniscient narrator who knows the characters’ thoughts, history and future evolution.

Two teenage best friends, working-class Julio and upper-class Tenoch, graduate from high school and see their girlfriends off on a trip to Italy. Despite their vow to remain faithful to the girls, they intend otherwise. Their plans for fun, however, deteriorate and they spend time smoking pot, swimming in a country club and at one point masturbating together on diving boards.

During the fatuous wedding reception of Tenoch’s older sister, they meet Luisa, the Spanish wife of Tenoch’s cousin Jano. Trying to impress her, the boys describe a fictitious secluded beach called Boca del Cielo (“Heaven’s Mouth”), to which they invite her. Days later she visits a doctor for some test results, and receives a drunken call from Jano who tearfully confesses an infidelity. The next day, she takes Tenoch and Julio up on their invitation.

Even though Julio and Tenoch know it to be an aimless trip, the three set off and drive through rural Mexico. The teenagers talk about their friendship and, when asked by Luisa, boast about their sexual exploits. Luisa speaks of Jano and recalls her first love, who died in a motorcycle accident. During one of their stops, Luisa leaves a message on Jano’s answering machine explaining that she has left him.

Throughout the trip, Luisa’s extroverted and upbeat persona is interspersed by bouts of inconsolable crying, some of which are accidentally witnessed by the boys. On one of these occasions, Tenoch enters her motel room in search of shampoo, at which point she daubs her tears and seduces him. Julio witnesses them having sex and, upset, walks away. He later tells Tenoch he had sex with Tenoch’s girlfriend, in violation of one of the rules governing their friendship. Tenoch spends the night furiously scolding Julio and asking for details.

Noticing tension the next day but still unaware of Julio’s confession, Luisa has sex with him to “equalize” the boys’ perceived status. A jealous Tenoch then reveals that he, too, has had sex with Julio’s girlfriend, sparking a quarrel that nearly comes to blows. After being shoved away by Julio while trying to intervene, Luisa angrily berates their immaturity and sexual incompetence, dismisses their rivalry as closeted homosexuality, and walks away. Shocked by her outburst, they beg her to stay, which she does in exchange for her own set of rules to keep them at bay.

They make camp near the sea and meet a local fishing family who boats them to an isolated beach, coincidentally called Boca del Cielo. They relax and enjoy the ocean, but upon their return find their campsite ransacked by a herd of runaway pigs. They spend the night in a nearby village, where Luisa makes another phone call to Jano to bid him an affectionate but final farewell.

Luisa, Julio, and Tenoch get drunk that evening and joke about their sexual histories. Julio and Tenoch reveal that they have frequently had sex with each other’s girlfriends, not just once as originally confessed. Julio adds that he had sex with Tenoch’s mother, but it is unclear whether he is serious. The three dance together sensually and then retire to their room. As Luisa kneels and stimulates them both, Julio and Tenoch embrace and kiss each other passionately.

The next morning, the boys wake up naked together. They bolt from bed and express a sudden eagerness to return home. The narrator explains the subsequent events: the boys’ journey back is quiet and uneventful, Luisa stays behind to explore nearby coves, the boys’ girlfriends break up with them upon returning from Italy, and the erstwhile best friends stop hanging out.

A year later, after a chance encounter in Mexico City, Julio and Tenoch go for coffee together. They awkwardly catch up on each other’s lives and news of their mutual friends. Tenoch informs Julio that Luisa died of cancer a month after their trip, and that she had been aware of her prognosis during their entire road trip. Tenoch finds an excuse to leave, and despite the two agreeing to meet up again, the narrator reveals that this is the last they will see of each other.