The film combines straightforward storytelling with periodic interruptions of the soundtrack, during which the action continues, but a narrator provides additional out-of-context information about the characters, events, or setting depicted. In addition to expanding on the narrative, these "footnotes" sometimes draw attention to economic/political issues in Mexico, especially the situation of the poor in rural areas of the country.The story itself focuses on two boys at the threshold of adulthood: Julio (Gael García Bernal), from a leftist middle-class family, and Tenoch (Diego Luna), whose father is a high-ranking political official. The film opens with scenes of each boy having sex with his girlfriend one last time before the girls leave on a trip to Italy. Without their girlfriends around, the boys quickly become bored.At a wedding, they meet Luisa (Maribel Verdú), the Spanish wife of Tenoch's cousin Jano, and attempt to impress the older woman with talk of an invented secluded beach called la Boca del Cielo ("Heaven's Mouth"). She initially declines their invitation to go there with them, but changes her mind following a phone call in which Jano tearfully confesses cheating on her.Although Julio and Tenoch have little idea where to find the promised beach, the three set off for it, driving through poor, rural Mexico. They pass the time by talking about their relationships and sexual experiences, with the boys largely boasting about their modest exploits, and Luisa speaking in more measured terms about Jano and wistfully of her first teenage love, who died in an accident.On an overnight stop she telephones Jano, leaving a "goodbye note" on his answering machine. Tenoch goes to her motel room looking for shampoo, but finds her crying. She seduces him, and he awkwardly but enthusiastically has sex with her. Julio sees this from the open doorway, and angrily tells Tenoch that he's had sex with his girlfriend. The next day Luisa tries to even the score by having (equally awkward) sex with Julio; Tenoch then reveals he had sex with Julio's girlfriend. The boys begin to fight, until Luisa threatens to leave them.By chance they find an isolated beach which coincidentally is called Boca del Cielo. They gradually relax and enjoy the beach and the company of a local family. In the nearby village, Luisa makes a final phone call to Jano, bidding him an affectionate but final farewell.That evening, the three drink excessively and joke recklessly about their sexual transgressions, revealing that the two boys have frequently had sex with the same women (their girlfriends, as well as Luisa). "Y tu mamá también," Julio jests to Tenoch. The three dance together sensually, then retire to their room. They begin to undress and grope drunkenly, both boys focusing their attentions on Luisa. As she kneels and stimulates them both, they gasp and kiss each other passionately.The next morning, Luisa rises early, leaving the boys to wake up together, naked. They immediately turn away from each other, and are eager to return home. The narrator explains that they did so quietly and uneventfully, but Luisa stayed behind to explore the beaches. He further relates that the boys' girlfriends broke up with them, they started dating other girls, and they stopped seeing each other.The final scene follows a chance encounter a year later, in 2000, the year that the Institutional Revolutionary Party lost the first election in 71 years. They are having a perfunctory cup of coffee together, catching up on each other's lives and news of their friends. Tenoch informs Julio that Luisa died of cancer a month after their trip, and that she knew she was ill the whole time that the three were together. Tenoch excuses himself, and they never see each other again.