Wild Grass, Alain Resnais, 105 mins (12A)

At the age of 88, most film-makers would be happy to sit back and enjoy the view. Not Alain Resnais, who has swerved into a wild new style of cinema

With film-makers of a certain vintage, you tend not to expect surprises – but you sometimes get them anyway.

Eric Rohmer, who died in January, aged 89, could have gone on making tart moral comedies for the rest of his career, but instead wound up his filmography with a spy thriller and a historical fantasy about swains and shepherdesses. And Portuguese prodigy Manoel de Oliveira recently unveiled his first venture into CGI – at the age of 101.

But Alain Resnais? Surely we knew where we were with the dependable, long-unfashionable Resnais. A radical innovator and fabricator of perplexing formal labyrinths from the 1950s through to the 1980s (among them, the legendarily enigmatic Last Year at Marienbad), Resnais seemed to be long settled into a comfortable theatrical rut. Under the influence of French boulevard theatre and our own Alan Ayckbourn, Resnais had spent more than 20 years exploring cinematic stagecraft in a way that sometimes yielded quietly impressive results, but occasionally carried a whiff of academic mildew (2003's laborious operetta Not on the Lips).

Then along comes Resnais's new film, and it turns out to be a meteor of out-and-out strangeness. Wild Grass is all the more peculiar because for much of the time it resembles a drily inconsequential French comedy of manners, and yet – just when you think you know what's what – it will start larking about in a very disconcerting manner indeed.

Based on Christian Gailly's novel L'Incident, the film is about an incident – an utterly banal one, at that – and its peculiar repercussions. Out shopping in Paris, dentist Marguerite Muir (Sabine Azéma) has her handbag stolen. One Georges Palet (André Dussollier) finds her purse, takes a shine to her ID photo and apparently falls in love with her – at least, stalks her with obscure intent. Then, for no clear reason, Marguerite falls in love with him, or appears to. Other characters get involved – Palet's wife, Marguerite's best friend, two policemen straight out of Tintin, or perhaps Kafka. Matters reach a bizarre airborne climax when amateur pilot Marguerite invites some of the other characters for a spin in her Spitfire ...

On paper, this may not look very promising – and the voice-over narrator (Edouard Baer, faithfully reciting extracts of Gailly's novel) admits as much. He's not even sure where this ordinary-seeming tale is taking us: "The commonplace and mundane can lead to .... To what? We'll see."

Nothing can be taken for granted, apparently, because the story is making its mind up as it goes along. Why do security guards stare at Georges as he passes? Why do the film's camera style and score (by X-Files composer Mark Snow) momentarily slip into the mode of a tense urban thriller? Is Georges, apparently an affable middle-aged loner, in fact a real or potential killer, or simply a bored fantasist? Who knows for sure? Neither Resnais nor his unseen narrator, it seems – but then "knowing for sure" is surplus to requirements in this kind of ever-shifting fiction.

The characters are so sketchy and inconsistent that they hardly qualify as characters in the normal sense. And the drama – or comedy, or whatever it is – is infuriatingly unstable. But what keeps us watching is the film's stylistic flamboyance. Wild Grass seethes with a mercurial brio not seen in a Resnais film for ages, recalling his Providence (1976), also about fiction and imagination. Resnais confounds us from the off, opening on the ominous and enigmatic image of a stone tower with a gaping dark doorway, followed by grass pushing through cracks in a pavement. Images of wild grass reappear throughout, counterpointed with the Palets' newly mown lawn – suggesting that desire and imagination, burgeoning uncontrollably, can't be tamed as easily as a suburban garden. Neither can the elaborate style developed by Resnais and cameraman Eric Gautier, who shoots much of the film in a version of 1980s soft-focus chic, lighting it in vivid boiled-sweet colours. Sets are crazily over-decorated: Marguerite's flat, with its neon sculptures, resembles an art gallery's stock room, mocking the very idea of realistic detail.

We're not watching an imitation of real life or anything like it, as Resnais reminds us when he hovers on a neon sign reading "Cinéma"'. There might be nothing very new in such meta-fictional winks, yet the overall result feels fresh and mischievous. Whenever we think we're on solid ground, Resnais flips us up in the air: literally so, the camera at one point vaulting gratuitously over the roof of the Palet house. Georges's interview with two policemen (a twitchy double act by Mathieu Amalric and Michel Vuillermoz) collapses in a flurry of repeated lines, cuts and abrupt zooms: a burst of old-school Resnais discontinuity.

A certain goofy whimsy may not be to all tastes – the crazy-haired Azéma can rather remind you of Felicity Kendal after sticking her finger in a light socket. But no one is better than Dussollier at playing harassed men of matured years, and he keeps us wondering to the end whether his Georges is sympa or an outright salaud.

What really makes the film, though, is its closing flourish – just when the budding romance seems to gel, Resnais and his runaway camera give matters a final enigmatic shake-up. The ending is as baffling as the last moments of 2001, and in its way, as eerie.

Wild Grass may not be among his very greatest films, but it's a magnificent rebirth, with Resnais proving more playful, more provocative, more downright youthful than he has been in years. At 88, film-makers aren't supposed to make comebacks this exuberant. Wild Grass is wild all right, and so, apparently, is the old master at its helm.

Next Week:

Jonathan Romney wonders whether two other veterans – Francis Ford Coppola and Woody Allen – can pull off Resnais-style comebacks