Julio (Bernal) and Tenoch (Luna) are two seventeen-year-old best friends whose sexy, young girlfriends are headed to Italy for summer vacation. At a family wedding in Mexico City, the boys' attention is diverted by Luisa (Verdu), a stunning older Spanish woman, who is married to a distant cousin of Tenoch's. The boys invite Luisa on a road trip with them to a remote beach called Boca del Cielo (Heaven's Mouth), neglecting to mention that they wouldn't know where to find it, even if it actually did exist. Needing a change of scenery, Luisa humours the boys and accepts their invitation.
During their trip, Luisa's presence brings out the best -- and the worst -- in Julio and Tenoch. They learn things about each they never knew, despite having been best friends for years, and Luisa, too, finds out what is important to her. As their journey progresses, all three companions find themselves confronted with their innermost demons and desires.