Julio and Tenoch are best friends that come from somewhat similar backgrounds in Mexico City. Julio is from a middle-class family with leftist political views, while Tenoch is from a wealthy and conservative political family. Their girlfriends, Cecilia and Ana, have both gone abroad for holiday. At a wedding of a member of Tenoch's family, the boys become infatuated with an older woman, Luisa, the wife of Tenoch's cousin Jano, and try to seduce her with a plan of showing her a secluded beach called Boca del Cielo ("Heaven's Mouth"). Little does Luisa know, but they have made up this beach.
While Luisa has no interest in taking the teenagers up on their offer, when she receives a message from her husband, Jano, while he is on a trip confessing to having an extra-marital affair, she impulsively calls Julio and Tenoch and asks them to bring her to the beach.
Even though they made the beach up, Julio and Tenoch are intent on getting some time with Luisa, so they pick her up and hit the road. In their travels, they talk about their relationships, and the boys discuss their code of ethics—a "Manifesto" adhered to closely by all of their friends. At one of their stops on the road, Luisa calls Jano and tells him that their marriage is over. Upset from the breakup, she lures Tenoch into her bedroom and convinces him to have sex with her. Julio sees this and gets upset, and as a way of getting back at Tenoch confesses to him that he had sex with Tenoch's girlfriend, Ana.
Tenoch is incensed at the revelation that Julio betrayed him, and the friends fight. Luisa realizes the boys are not talking to each other and thinks its because of her having sex with only one of them, so she has sex with Julio to even things out. Immediately afterwards, Tenoch reveals he too has slept with Julio’s girlfriend and the boys fight even more.
When the fight escalates, Luisa threatens to leave the trip and walks away, but the boys pick her up and get her to agree to continue on. Somewhat by accident, they find themselves at the beach, where they meet Chuy, a fisherman, his wife, Mabel, and their children, who take them on a tour of the surrounding beaches. One of the beaches is, miraculously enough, called "Heaven's Mouth."
That night the trio gets very drunk. Tenoch and Julio each confess to sleeping with one another's girlfriends multiple times, but they harbor no hard feelings about it. The group continues drinking until Luisa seduces them both and they all go back to her bedroom. There, they all have sex, with Julio and Tenoch kissing and engaging with each other in the process.
The next morning when the boys wake up, they are embarrassed by what they did and plan to go home. Luisa stays behind to spend time with Chuy and Mabel's family. When Julio and Tenoch return home they no longer speak to each other.
A year later the friends bump into each other and Tenoch reveals to Julio that Luisa died one month after they returned from their trip from cancer, knowing the whole time that she was going to. Tenoch leaves and the narrator reveals that the two friends will never see each other again.
