The film intricately weaves straightforward storytelling with intermittent narrative commentary, enhancing the viewers' understanding of the characters and their surroundings. Throughout the film, a narrator occasionally interrupts the action, offering context and shedding light on significant social and political issues affecting Mexico, particularly the struggles faced by the impoverished in rural areas.
At the heart of the story are two boys on the cusp of adulthood, Julio (Gael García Bernal), who hails from a leftist middle-class family, and Tenoch, whose father holds a prestigious political position. The narrative opens with each boy experiencing one last intimate moment with their girlfriends before the girls embark on a trip to Italy. Left to their own devices, the boys soon find themselves bored and restless.
Their fortunes change at a wedding, where they encounter Luisa (Maribel Verdú), the Spanish wife of Tenoch's cousin Jano. Eager to impress Luisa, they spin tales about an elusive beach known as la Boca del Cielo ("Heaven's Mouth"). Initially, she declines their invitation to join them, but a conversation with Jano—who tearfully confesses his infidelity—nudges her to reconsider.
Despite being uncertain about the beach's actual location, the trio embarks on a journey through the scenic yet impoverished landscapes of rural Mexico. During their travels, engaging conversations arise about love and sexual escapades, with the boys boasting about their youthful experiences while Luisa reflects nostalgically on her first love, who tragically died in an accident.
During a night at a motel, after leaving Jano a "goodbye note" on his answering machine, Tenoch encounters Luisa in a vulnerable moment. She seduces him, leading to a rushed and clumsy sexual encounter that Julio unwittingly witnesses. Conflict arises as Julio, feeling betrayed, confronts Tenoch about the situation. In a bid to reclaim her agency, Luisa subsequently has a similar tryst with Julio, igniting further tension as they realize that both have engaged with the same woman.
By a stroke of luck, they eventually stumble upon the secluded Boca del Cielo beach which embodies the freedom and beauty they sought. Their initial awkwardness gives way to relaxation, and they find joy in the company of the local inhabitants. In a poignant moment, Luisa places a final phone call to Jano, saying a heartfelt goodbye.
As the sun sets on their adventures, the trio indulges in heavy drinking and playful banter about their romantic entanglements, with Julio quipping, "Y tu mamá también," referring to their shared experiences. This leads to a charged moment where all three engage in a sensual dance and mutual exploration, showcasing a blend of youthful exuberance and vulnerability.
The next morning, Luisa rises early, leaving Julio and Tenoch in bed together, both naked and immediately distancing themselves from one another. They are eager to return home, and the narrator notes that their return was uneventful, while Luisa opted to stay behind and embrace the beauty of the beaches. In the aftermath, the boys face the consequences of their choices as they break up with their girlfriends and drift apart from each other.
In a somber twist, the film's conclusion brings the two boys back together a year later, in the year 2000, amidst a changing political landscape in Mexico. While sharing a casual cup of coffee, they catch up on their lives and the fates of mutual friends. It is here that Tenoch reveals the heartbreaking news of Luisa’s death from cancer, a battle she carried during their time together, leaving an indelible mark on both boys and their shared memories. After this revelation, Tenoch departs, signaling an enduring estrangement and the end of their once close friendship.
