“What a bunch of assholes, huh?” asks Julio, slouching up to the bartender at the wedding of the president’s daughter. The fact that Julio, despite his suit and his position on the other side of the bar, is a couple of class rungs below the Mexico City aristos around him—that would be El Presidente, as in of the country—isn’t a distinction the working man knows or cares about. Details such as this fly by in Alfonso Cuaron’s brilliant new film, Y Tu Mama Tambien, whose characters subtly and completely shade in a tapestry of class distinctions and their limits while talking nonstop about sex.

Sexual union—depicted, discussed, and imagined—turns out to have much more give than social standing; after all, liaisons come and go, but the hierarchy lasts forever. Julio (Amores Perros’ Gael Garcia Bernal) and Tenoch (Diego Luna) have just graduated from high school; they’re ready for some kind of big adventure but are too content with aimless indulgence to imagine what form that adventure might take. The middle-class Julio and richie Tenoch, a classic preppie stoner who’s the son of a cabinet minister, have an exultant, deeply intertwined friendship characterized by exaggerated sexual bravado and childish “rules” and rituals, fueled by the recklessness their comfortable backgrounds make possible.

Periodically, as the boys bounce from a foggy teenage drug party to the sunny pampering of the wedding reception, the film’s sound cuts out and cold narration takes over, filling us in on peripheral characters’ stories and preparing us, but not the unwitting young men, for some overwhelming future. We’re told what they share with each other, but we’re also told what they don’t—their teenage primness, their childish attachments, the tiny class complications wedged into hidden corners of their friendship.

Bored with making mischief at the wedding, the boys impulsively light upon a beautiful stranger, who turns out to be the wife of Tenoch’s pompous writer cousin. An idle conversation about the boys’ summer plans escalates into an invitation for the woman, a Spaniard named Luisa (Maribel Verdu), to join them on a road trip they never intend to take to a beach that does not exist. “Heaven’s Mouth,” they convince her, playful invention bouncing between them like the reflex it is, is a deserted paradise, where the sand is white and fine and the water always warm. The interlude is forgotten until Luisa, whom we follow home, receives two things—a very funny drunken phone call from her jerk husband and a report from a doctor’s office, which is the only bum note in the film. Plunked at a crossroad, Luisa calls up the boys and asks if the trip is still on.

Driven by hormones and the unspent energy of youth, Tenoch and Julio have no plans more concrete than somehow seducing the lovely older woman along the way—after all, they don’t know where they’re going. Luisa’s motives are more enigmatic. She’s delighted by the endless supply of 40-ounce cervezas and fat joints, and as the trio makes its way vaguely north, she teases and interrogates the boys about their sex lives; but in the shabby motels where they stop, she cries alone in her room. The sexual tension grows lusher and less defined, the boys’ increasing competition with each other nearly indistinguishable from their unresolved desire. Finally, Luisa invites Tenoch to bed, then seduces Julio in the back of the car; but the boys find that since they left home, they’ve grown just enough to render this hormonal fantasy fulfillment complicated and painful. Fault lines in their friendship split and gape, and Luisa, who’s running on impulse alone, can’t reconcile them in any meaningful way.

But it isn’t the sex that drives them apart—it’s the realities of their future, realities almost inevitable in a society of rigidly defined class roles. Cuaron has great fun taunting upper-class Mexico at the beginning, showing Tenoch’s slick spiritual mother negotiating the steppingstones on her flawless lawn. This New Capitalist Mexico, with its pale-skinned aristocrats temporarily seized with nationalistic fervor, produces stark absurdities such as expensive, organic-looking country-club furniture right out of the Moon Age ’60s architecture Jacques Tati used to send up and mansion-dwelling children with Aztec names. At the wedding reception, his camera follows a high-heeled servant out to the parking lot, where she delivers a tray of exquisite canapes to the dozens of beefy, black-suited bodyguards draped over shiny silver luxury cars.

As the journey progresses, the depiction of what politics and money are doing to the economically weakest segments of the country grows darker and more serious. The cool voice-over details the fate of a fisherman’s family as a luxury high-rise is built in his village; Tenoch, Julio, and Luisa laugh and prattle over sexual minutiae while driving serenely past checkpoints at which poorer, therefore more defenseless, citizens are being searched. The boys lay out their decadent, schoolboyish 10-point “manifesto” for Luisa, but later she comes back with a harsh, dictatorial one of her own.

Y Tu Mama Tambien—the phrase, roughly equivalent to the dated insult “So’s your mother,” is both punch line and throwaway joke in the movie—is directed in a bravura series of long, unbroken scenes in which the energy and chemistry among the characters feel utterly natural and the camera never cuts away. The rapid-fire dialogue is brilliantly translated by Timothy J. Sexton, whose choice of insults and idioms (there are many of both) has as much veracity as it does immediacy—you never feel as if you’ve missed a thing, and he’ll trade a puta madre for an “asshole” if a direct translation might muddy the sense. Luna and Bernal are astounding as the two friends, their youthful fearlessness exhilarating and their grownup hesitancy heartbreaking. (The actors appeared together as children on a Mexican television series.) Cuaron’s script falters only in giving Luisa a handy Hollywood reason for embarking on this wild ride—it would have been just as easy for her to be swept away, as we are, by the jouncing puppyish energy of Tenoch and Julio’s passionate drive to connect with anything and everything before they have to discard childish things.CP