The Romance of Astrea and Celadon, 12A

At 88, Eric Rohmer, doyen of New Wave cinema, blends sex and sobriety in one of his most surprising films yet

There are some films that not everyone will get. It's not because they're difficult, or require some arcane key to unlock their mysteries. It's simply that you either get them or you don't, just as you might or might not take to a tune or laugh at a joke: if you don't warm to the spirit of the thing, you may as well forget it.

You might be immune to such a film because the premise strikes you as ludicrous, and the execution preposterous. And you might understandably feel that way about The Romance of Astrea and Celadon, by the veteran French director Eric Rohmer, the doyen of the New Wave directors. No doubt you have to wait until you reach Rohmer's age to make a film so downright outré: when you're 88, no one's going to tell you not to.

With Rohmer films, you used to know what you were going to get: contemporary comedies of manners such as My Night With Maud, the Four Seasons series, et al. Astrea and Celadon, however, is a startlingly esoteric project: a romance set in a bygone pastoral Arcadia in which young shepherdesses tend their flocks in billowing gowns, and swains bemoan lost loves while wearing flowery straw hats. Not to put too fine a point on it, they dally amorously awhile, all but singing hey-nonny-no.

Rohmer's source is L'Astrée, an immensely prolix 17th-century novel by Honoré d'Urfé – one of those classic texts that even the hardiest academics shudder to read. Opening captions explain the film's intention concisely. The novel takes place in fifth-century Gaul, but Rohmer regrets to inform us that he was unable to shoot in the Forez region, D'Urfé's original setting, because it's been spoiled by modern building and agriculture: he had to find other locations that still resemble L'Astrée's bucolic ideal.

So what Rohmer is doing in this film – shot largely in the open air amid fields, woods and many a verdant bower, tra-la – is evoking a 17th-century vision of an imaginary fifth-century France. But the no-frills cinematography applied to stylised, formal action reminds us that we're watching 21st-century actors performing an anachronistic masquerade. As a result, Rohmer has no need to frame his film archly: he can play it straight, respecting the material and its conventions. An English-speaking audience faced with, say, Twelfth Night filmed outdoors, in period costume, wouldn't consider it weird; there's no a priori reason to find Astrea weird either. But Rohmer's film undeniably is weird – and the weirdness largely stems from the logic of the narrative, a logic presumably true to the novel.

The intrigue begins with the hero, Celadon (Andy Gillet), upsetting his beloved Astrea (Stéphanie Crayencour) by dancing with another at the bagpipe-heavy village hop. Aggrieved, Astrea sends him packing and Celadon does the correct thing for a Gaulish lad of integrity, which is to fling himself into the nearest foaming torrent. He is rescued by the nymph Galatea (Véronique Reymond), who falls for this sodden hunk and carries him off to her castle. She intends to keep him there, too – nymphs in this kingdom enjoying an autocracy that befits one such as Galatea, a haughty odalisque with a distinct resemblance to Hilary Swank and a decided swank in her gait. Even so, Celadon makes his excuses and wanders off to haunt the forest, sleeping on ferns, writing many a rueful poem, building a rough-hewn cabin, all considered highly therapeutic activities back then. Eventually, Adamas (Serge Renko), a wise and waggish druid, suggests that cross-dressing might be the answer to Celadon's problems.

This may not sound like any Rohmer film you've seen, although the director has previously ventured off-piste in a handful of historical dramas (eg, 1978's Perceval le Gallois), albeit never so eccentrically. Narratively, though, the film is pure Rohmer: a comedy of manners, about a couple entangled in a dilemma arising from the moral codes of their day, and reunited after intricate detours.

Rohmer's films have always been about sex in a subtle, almost sublimated way: Claire's Knee made something perversely discreet of its hero's peculiar anatomical fetish. But Astrea and Celadon is by far his most overtly sexual film: what with the women's diaphanous off-the-shoulder (way off-the-shoulder) frocks, this is surely the first Rohmer film with a nipple count. But the sex operates within the artistic codes of the period. At one point, Celadon finds Astrea asleep, her dress hiked up just so: sure enough, we see a perfectly 17th-century thigh, straight from a Poussin painting. Indeed, Crayencour altogether resembles a period portrait, her marble rosiness neatly framed by a neo-classical coiffure.

Once Celadon gets into drag as a druid damsel, the film shifts into a bizarrely perverse, but somewhat Shakespearean comic register. You can't imagine the pretty but square-jawed Gillet being remotely convincing as a woman, and indeed, with his wimple and wavering falsetto, he'd be quite at home in an Almodóvar film about transsexual nuns. Yet Astrea and her girlfriends buy the deceit, and when they and Celadon first share maidenly giggles in the bedchamber, I couldn't help thinking of the impromptu cocktail party hosted in a sleeping car by Jack Lemmon's "Daphne" in Some Like It Hot. Come the surprisingly erotic pay-off, you could almost be watching a meeting of the Sapphic Society at Malory Towers.

Rohmer knows when he's being over-the-top – he simply chooses to be so in ways befitting the codes of pastoral. Hence the wild mugging of Rodolphe Pauly as the minstrel Hylas, a cartwheeling jackanapes who's forever plucking his lute while laughingly expounding his credo of carefree promiscuity. There are also moments of high seriousness in a scholarly vein: notably the scene in which Adamas holds forth, in anachronistically Catholic mode, on Gaulish theology (you'll no doubt remember the god Toutatis from your Asterix comics). And throughout characters learnedly interpret the various poems, inscriptions and allegorical paintings that Celadon has left behind him like clues in a semiological Easter egg hunt.

Astrea and Celadon may be barmy by conventional standards, yet it's barmy in an august, controlled way that bespeaks the hand of a master film-maker. It also has a modest, understated beauty that flies in the face of most contemporary costume drama. And, while your jaw may be dropping for much of the film, overall there's an emotional directness that harks back to silent cinema. It's quite possible that you won't warm to Astrea and Celadon, not remotely. But if you do, then it'll be hey-nonny-no all the way.