There's not supposed to be anything easy or straightforward about a new film from octogenarian director Alain Resnais, and Wild Grass does not disappoint. Premiering at the 2009 Cannes Film Festival just a couple of weeks before the filmmaker's 87th birthday, the film shows no sign of his age: it's maddening and challenging and frequently brilliant, a movie that engages with cinematic vocabulary in offbeat, casually thrilling ways, at a time when even "art" movies are typically happy to toe the line of convention. Parts of it are befuddling, but it's a damn privilege to be befuddled by a film that has been put together with this much insight and precision.

The simplest place is to begin with the plot, though it's not the best place. But disregarding that: a dentist named Marguerite Muir (Sabine Azéma) has her purse snatched, and a little while later a man named Georges Palet (André Dussollier) finds it in a shopping center parking garage. He tries to contact her, and failing that, turn the wallet in at the police department; when she calls to thank him, he is impossibly rude, and his attempts to apologise turn into a frightful, stalkerish series of letters and calls and tire-slashings. Marguerite eventually calls the same police department to make him stop, but then has a change of heart, and thereupon tries to re-enter his life.

Not the best angle to approach the film, like I said, but at least it gives us something to hang on to. Like other Resnais films, Wild Grass uses its narrative as a tool to explore what I would hesitantly call "models of knowing" - there are only vague ways of describing what I'm thinking of, but several of his best-known works investigate the ways that we perceive and compartmentalise knowledge, which in a Resnais film is always incomplete and faulty. The perversity and genius of this particular movie (perversity is genius here) is that every time we think we have a handle on a character or a situation, the director and writers Alex Reval & Laurent Herbiet (adapting Christian Gailly's novel L'incident) change the rules of the game on us. Not capriciously: other than the odd changes in Marguerite's feelings from fear and loathing to guilt and the desire for reconciliation, we're never really aware when the ground shifts beneath our feet.

Deprived of a baseline reality - it wouldn't be unreasonable to accuse the film of incoherence, if only it weren't so deliberate - Wild Grass has a fragmentary, almost schizophrenic quality that mirrors the erratic minds of its two protagonists. The title is very nearly the right translation of the French Les herbes folles, though a more literal rendering would be "Crazy Grass", and the distinction is important: there is a singular lack of sanity in the world of the film, obviously in the case of Georges (who fantasises about killing young women in certain scenes, and tries to rehearse his first conversation with Marguerite in a ballsy double-exposure), only slightly less obviously for Marguerite (frequently costumed in a squirrelly, button-heavy suit that, combined with her explosive nest of red hair, makes her look like a drum majorette with her head on fire), and even in the case of the film's designated "normal" people, Georges's wife Suzanne (Anne Consigny) and Marguerite's colleague Josepha (Emmanuelle Devos), though in a subtle way.

Wild Grass isn't exploring insanity, though, not in such bald terms: it's an effort to drag coherence out of incoherence, that keeps failing because the movie always insists on darting away - it's the cinematic equivalent of trying to catch a live fish with your bare hands. Especially in its final handful of scenes - especially in its final shot - Wild Grass is a study in mutability and the potential non-existence of solid identity that puts the blockbuster dream-thriller Inception to shame. It's slippery in every way possible: from the very insistent way that Resnais keeps us from seeing Marguerite or Georges in their first scenes, accompanied by a warm-voiced narrator (Edouard Baer) whose grasp of the story is no greater than ours, though he seems entirely amused by his own inability to keep track of details, "um"-ing and backtracking to correct himself with the hint of laughter in his tone.

While all of this narrative fragmentation is going on, Resnais plays a two-handed game with the film's visuals, giving us certain cues to anchor ourselves - repeated slow-motion shots of Marguerite's purse in the hands of the thief or a field of the titular wild grass, for example - while using unconventional framing and other idiosyncratic visual tricks to leave the film mired in its own anti-reality. Sometimes, he combines these things: there is a magnificently sophisticated use of color in the film, especially red, yellow, and blue, that I'm not in the least prepared to analyse after just one viewing, though it provides a structure to the film while also giving a cryptic, dream-like aura to much of the mise en scène: the weird lighting in Marguerite's apartment, for example, which is a clear visual cue of her disconnect from normalcy long before the script has given us any real reason to suspect such a thing.

Wild Grass is unconventional to the point of dysfunction, yet Resnais finds a way to make the film's apparent incoherence the very reason that the film is a triumph. The film is about incoherent people, after all, though "irrational" might be a better word for it. The theme, if I can try to pin down such a wildly divergent movie, which cuts from goofy comedy to thriller to romantic drama in the space of a few shots, sometimes, relates to the fragility of "the normal", and it's rather messy, violent and confusing in the process, though also enthralling. It's hard to say whether the film is an example form following content, or content being dictated by form, though I suppose it's likeliest that such a distinction doesn't matter. What is certainly the case is that Wild Grass is a lingering film, and trying to capture its essence in a review is the height of folly - I look forward to revisiting and rethinking this film often in the future, which is rare praise indeed for any movie from any country in a period as bound by strictly coded filmmaking rules as our own. This is why Resnais continues to matter some six decades into his career: he still finds new ways to push the artform in the pursuit of exploring the corners of human perception.