La Nina Santa Lucrecia Martel 106 MINS, 15
The theremin - which famously produced the unearthly oo-ee-oo sound on "Good Vibrations" and countless sci-fi movies - is an instrument played by moving one's hands in the air around two antennae. "Because there is no physical contact with the instrument," explains website, "playing the theremin requires precise skill and perfect pitch." There couldn't be a more apposite figure for the craft of Argentinian director Lucrecia Martel, whose new film La Nina Santa ("The Holy Girl") displays just such unerring lightness of touch. Martel creates unsettling resonances while only rarely laying her fingers on the keyboard of conventional storytelling. If you think this metaphor's too fanciful, rest assured that the theremin does feature centrally in the film. It's during a recital that the film's teenage heroine feels a middle-aged man rubbing against her bottom: a bizarre juxtaposition of non-contact and improper-contact activities.

La Nina Santa, Martel's follow-up to her impressive 2001 debut La Cienaga, appears at first to deal in familiar, even overworked art-film territory: sexual awakening, religious fervour and the uneasy tensions between a young girl and a much older man (a theme that has pretty much kept French cinema bankable over the last three decades). Yet nothing in the film is quite clear: everything blurs, or shifts pitch as teasingly as the theremin howl itself. The setting is a run-down spa hotel owned by the family of divorcee Helena (Mercedes Moran). Her 16-year-old daughter Amalia (Mara Alche) is undergoing a sort of religious crisis at school, where teacher Ines (Ma Maestro) instructs the girls about the nature of spiritual vocation. Her pupils are rapturously enthused by the idea, but religious and sexual fulfilment, not unusually, are somewhat fused in their imaginations, not least because of the girls' fascination with the "pre-marital relations" that their glamorous teacher might be enjoying.

Amalia seems to hear her own other-worldly summons in the theremin's wail, which brings her, literally, up against Dr Jano (Carlos Belloso).

He happens to be attending a medical conference at the hotel, and notwithstanding his penchant for teenage frottage, he takes a shine to Helena too, the minute he glimpses her through a doorway, magnificent in a backless dress.

Part of Martel's subtlety is that she doesn't, as you might expect, focus entirely on Amalia, but shifts between several figures and their desires: Amalia, Helena, Jano and Amalia's friend Josefina (Julieta Zylberberg), who is making her own tentative but enthusiastic explorations in pre-marital relations. La Nina Santa focuses on all these characters, or none, which is its defining quality.

Visually, Martel and cinematographer Felix Monti take everything from an angle: the compositions are often skewed, diagonals running across the screen, but in a way that feels gentle and organic, rather than an obvious attempt at modernist starkness. The most important details are often fleeting, marginal: Jano nervously hesitates to touch Helena's waist as they have their photo taken, but this gesture takes place right at the edge of the frame, where it could easily be missed. One of the film's strangest close-ups catches a stray dab of shaving foam on Jano's neck, before Amalia whisks it away for further fetishistic study.

Much of the film's fascination is in how much eludes us at first viewing. For example, we can't easily define the relationship between Helena and the elderly woman, Mirta, who seems to actually run the hotel while Helena wearily hones her vampish elegance. Nor do we immediately get the connection between Helena and the middle-aged Harold Pinter lookalike who appears to be her lover, but is later identified as her brother. Similarly, we often hear people's voices before we see who they belong to: part of a soundtrack strategy that uses layered voices and noises off to keep us constantly alert.

Other obscurities and non-sequiturs are out-and-out head-scratchers. A sudden crash heralds the dreamlike apparition of a naked man outside Josefina's apartment; we later learn he has fallen two floors from his apartment, and then no more is heard of him. Most daringly of all, the film leaves us dangling while the one explosive moment towards which everything seems to be heading is tantalisingly withheld.

Throughout, Martel keeps bringing us back to the torpid, enclosed world of the hotel, a genteel sphere of enforced conviviality from which there seems as little possibility of escape as from Chekhov's crumbling country estates. The fact that the hotel's core is a naturally-heated pool sums up the peculiar balminess in which the film bathes. The overall look, with its faded 1970s haziness, stresses blues and caramel browns, playing Helena's tanned glamour against the hotel's claustrophobically drab warmth and odd echoes of Soviet functionality.

Martel casts distinctively, with an eye for striking ordinariness. Though one character remarks on her beauty, there's no glamour in Amalia: young actress Maria Alche is slightly doughy-faced, with a sullen look that occasionally breaks into conspiratorial smirks. Carlos Belloso's Jano is a shifty, unprepossessing, yet not entirely repellent figure. It's perhaps because he so obviously looks the part of the seedy middle-aged lecher that Martel is able to defuse the stereotype and make him into something more complex, the hub of a bitter comedy of misunderstanding.

All the charisma in the film rests with Mercedes Moran's Helena, the epitome of the woman who's seen the world, whose history - and future - remain unclear, but whose heavy, weathered looks suggest an intense but narcissistic sultriness. It's notable that in a film ostensibly about teenage libido, it should be the mother who is out-and-out sexy - suggesting that we're seeing things through the eyes of Amalia, who perceives her mother's sensuality as a model and a mystery.

All this vagueness and atmospheric density make La Nina Santa considerably more than a film about sexual awakening, or spiritual yearning, or the equivalences between the two; in the end, you can't be sure quite what it's about, and that is its glory. This richly perplexing film confirms Lucrecia Martel's status as one of the most intriguing new talents in world cinema, and it's another film that should boost the currently buoyant profile of Argentinian cinema. I'm not sure, though, whether it'll do much for public awareness of the theremin - Martel's shooting style is so elliptical that at first glance, I was convinced I was hearing a musical saw.