Is there a cinematic genre from which less is expected in terms of intelligence, originality or general artistry than the contemporary romantic comedy? OK, not counting porn, snuff films or anything involving Larry the Cable Guy. Here’s a movie form that, in a generation, has devolved from Annie Hall to Fool’s Gold. Can this possibly be good news for anyone besides Kate Hudson?

Apparently nobody got around to telling first-time director Marc Webb or screenwriters Scott Neustadter and Michael Weber that they weren’t obliged to produce anything more than 90 minutes of mindless fluff. (500) Days of Summer breaks all the rom-com rules: It’s smart, it’s funny, it’s charming; it turns conventions inside out, and all without ripping off Judd Apatow even once.

The film stars Joseph Gordon-Levitt as Tom and Zooey Deschanel as Summer. (Fun fact: Deschanel went to high school with Kate Hudson.) He’s a one-time student of architecture whose life path has taken him, curiously, into the greeting-card business. He has a Cyrano-esque knack for putting into words emotions others feel but can’t express. She’s the new assistant his boss has just hired. At least, that’s who she is on Day 1 of the titular 500.

By Day 500, she’s the woman Tom has loved and lost, to his utter bafflement. This isn’t a spoiler — fear not — but the movie’s well-advertised premise. The opening scene lays out the young man’s dilemma. He’s just spent the happiest 500 days of his life. Those days, weeks and months gave him zero reason to doubt that they were equally idyllic and meaningful for Summer. Somewhere along the way, however, she began to lose romantic interest. Nobody cheated. Neither committed any unforgivable offense against the other. None of the crimes that customarily send relationships off the rails in romantic comedies were involved. Love just suddenly stopped, like a motion picture when the film snaps in the projector.

Devastated, Tom is determined to rewind the romance in his memory for the purpose of pinpointing the moment when things started to go wrong. The filmmakers understand that the human brain — certainly the broken heart — does not replay events in an orderly, chronological fashion. Hence the film bounces around in time, offering glimpses of Tom’s 500 days with Summer in an order that has its own logic — beginning with Day 488, say, then looping back to Day 1. It casts about in time for clues to a mystery with no solution — at least, none of the kind one is conditioned to expect by decades of formula.

Zigzagging through the past with Tom is a hoot and a half for any number of reasons. Both he and Summer are exceedingly appealing characters, and both are brought to life by talented performers at the top of their game. Deschanel does an exemplary job, for example, of making Summer a bright, lively object of desire without resorting to traditional tropes of indie quirkiness.

The source of her fascination (aside from the actress’ innate off-kilter sparkle) is a clever ploy on the part of the film’s writers to reverse the standard male-female romantic-comedy roles. Gordon-Levitt is the half of the couple who takes things seriously; who develops a Valentine’s-Day-card vision of a life together forever; who clings.

Deschanel, by contrast, is convincing as a young woman who knows what she wants and is perfectly content to live for the moment. It’s not in her character’s nature to project into the future or define herself in terms of her current relationship. There’s a telling scene in which she shares her philosophy with one of Tom’s friends, and his reaction is perfect: “You’re a guy!”

The writing is smart, the musical choices serve the movie admirably, the cinematography treats downtown L.A. as lovingly as any Woody Allen picture has treated Manhattan, and Webb proves himself an innovative arrival. In one of the film’s most effective sequences, the director goes split-screen. On the left is Tom’s romanticized memory of a party at Summer’s place. On the right is the same series of events the way they actually happened. The device is not merely inspired, it’s funny as hell.

The same is true of (500) Days of Summer as a whole. It’s a breath of fresh air in a genre that stopped showing signs of life a long time ago. Its jilted hero may never figure out precisely what went wrong. Audiences, on the other hand, are likely to be impressed by just how much in Webb’s feature debut goes right.