Y Tu Mamá También (18)

Alfonso Cuarón's Y Tu Mamá También (And Your Mother Too) must be the most sophisticated teen sex comedy ever; it's certainly the most lyrical. Its raunchy theme – two teenage boys on an amorous road trip with an older woman – has given it a reputation as a Latin American take on the US wave of teenage gross-out farce. It has been widely dubbed Mexican Pie, but given the film's distinct art-house tone and the melancholic colour of its sexual shenanigans, Last Tango in Guadalajara might be more accurate.

Alfonso Cuarón's Y Tu Mamá También (And Your Mother Too) must be the most sophisticated teen sex comedy ever; it's certainly the most lyrical. Its raunchy theme – two teenage boys on an amorous road trip with an older woman – has given it a reputation as a Latin American take on the US wave of teenage gross-out farce. It has been widely dubbed Mexican Pie, but given the film's distinct art-house tone and the melancholic colour of its sexual shenanigans, Last Tango in Guadalajara might be more accurate.

Ostensibly, this is a ribald coming-of-age tale for boys. Cuarón's heroes are two boisterous teenage slackers, Tenoch (Diego Luna) and Julio (Gael García Bernal, the doomed young lover in the other recent Mexican hit Amores Perros). Unlike their counterparts in US high-school comedy, these boys are not gauche virgins desperate to get laid. In fact, they're more than used to sex, as we see in the opening scenes when they bid their girlfriends a demonstrative if rushed send-off before the holidays. The boys settle in for what looks like a dull summer – getting stoned, reluctantly turning up for family events, or jerking off into the swimming pool to dreams of Salma Hayek.

Things take a turn when Tenoch's novelist cousin arrives, accompanied by his beautiful Spanish wife Luisa (Maribel Verdú). At first, the boys gamely try to impress this elegant, aloof European newcomer, spinning her a yarn about their planned visit to an idyllic beach. They're astonished when she decides to join them, and altogether dumbstruck when she preempts their wildest dreams and starts making moves on them. Needless to say, the trio's sexual idyll proves messier and more complex than they expect, and more of a character-building experience too. This is one of those films – memorably mocked by Christina Ricci in The Opposite of Sex – which could easily contain the line, "After that summer, nothing was ever the same again." But Y Tu Mamá También is a genuinely surprising film, and not only because it's raunchier and more idiosyncratic than you might have expected from the director of mainstream Hollywood ventures A Little Princess and Great Expectations. There's more to it than suggested by its ostensibly off-the-peg lads' fantasy. As it turns out, the film is just as much about all those other things going on around the characters, of which they remain blissfully unaware. In a bold stylistic move, the sound keeps cutting out to be replaced by the voice of an omniscient narrator, who seems to tells us more than we need to know: who cares whether the father of one of the boys' girlfriends is a Lacanian psychoanalyst? But then we start to discover other contingent facts to which the boys are oblivious – the death of a migrant bricklayer, or an accident on the road 10 years before their trip. We also start noticing events that the boys miss entirely, such as a roadside arrest. Gradually, the wider picture behind their pampered, callow existence fall into place: Tenoch's politician father, the narrator casually tells us, was involved in a contaminated food scandal. The film shows us far more of Cuarón's country than the touristic colour we initially think we're getting: despite a mariachi wedding early on, this is as hard-edged a study of modern Mexico as the overtly gritty-realist Amores Perros.

As the journey develops, we realise we are watching a picaresque tale about know-it-alls who really know next to nothing. Tenoch and Julio are bound by membership of their own laddish secret society of "Charolastras" (self-styled "astral cowboys"), but it takes Luisa's company to unearth just what they have failed to realise about each other. They also fail to understand just why she has signed up for the journey; it finally comes as something of a shock when we learn that we have missed the point as well, giving into the story's promptings to overlook its darker aspects. The final revelation at first looks somewhat retrograde in its sexual politics, but what we are actually getting is a Mexican update of a French 19th-century novel: the closing scene, all the more poignant for its restraint, echoes the ironic bathos with which Flaubert signed off Sentimental Education. Early on, we learn that Tenoch aspires to be a novelist: but for that you need life experiences, his haughty white-suited cousin sneers. We emerge from the film with the nagging suspicion that we have actually been watching the novel that the older, wiser Tenoch eventually writes.

This account may make the film sound more melancholic that it actually is. In fact its sense of life and adventure, both risky and risqué, is astonishing. That's partly because the film, like its characters, explores the entirely untouristic backroads of a little-seen Mexico, and partly because Cuarón and cinematographer Emmanuel Lubezki shot their long leisurely takes chronologically: you can feel the trio's relationship unfold from mutual embarrassment, through tense power games, through to the payoff where Julio and Tenoch get about as much in the way of life experiences as they can quite handle.

The boys are magnetic, two personable TV soap graduates playing it young, dumb and full of come, backchatting away in Mexico City slang, but gradually unearthing the confused vulnerabilities beneath the stoner machismo; one roadside shot has a distraught Tenoch, in shorts, turning back into a lost little boy before our eyes. The toothy, gangling Maribel Verdu has a more awkward role to carry off – she has to convince us that there is more to Luisa than the boys suspect, but she convinces better as their teasing, conspiratorial sex buddy, and as a rule-making reluctant governess, than when giving us fleeting intimations of suffering and collapse. But the awkwardness of Luisa's character is at least an index of the risk Cuarón takes by packaging melodrama in the entirely incongruous genre of teenage sex comedy. You don't often see a film that manages to be so lyrical, so moving, so troubling in its sexual dynamics – and so bracingly rude into the bargain.

j.romney@independent.co.uk