For a lot of casual filmgoers in their teens and 20s -- the ones yet to encounter a Charlie Kaufman script such as "Eternal Sunshine of the Spotless Mind" or who haven't seen the bittersweet 1967 "Two for the Road," written by Frederic Raphael, or have yet to dive into a Milan Kundera novel -- the structural mind games played by the romantic comedy "(500) Days of Summer" may throw them, happily, for a loop.

I hope so. The structure's mainly what this pleasant summer-weight diversion has going for it. The film gets by on a few funny lines (from the screenwriters of " The Pink Panther 2," which is a few more than "The Pink Panther 2" offered); some playful split-screen imagery from first-time feature film director Marc Webb, who made creative, witty videos for Green Day, All-American Rejects and Weezer; and the real star of the picture, second-billed: Zooey Deschanel.

Joseph Gordon-Levitt plays the lead, Los Angeles greeting card writer Tom, whose heart gets kicked around in an improving way by Summer, a free spirit played by Deschanel. Gordon-Levitt is a good actor. But Deschanel, playing his lover/tormentor/sphinx/minx, a character one dimension short of three-dimensional, is more interesting. Her timing isn't like anyone else's; just when you think she's coasting on her charm, she'll sharpen the pace and bring out something fresh and seemingly improvised in her co-star, and you feel as if you're watching a real relationship.

The gimmick is simple. Tom meets Summer at work, and though they hold opposing views on love -- he's the open-hearted romantic, she's the hide-of-steel "alpha male" -- they fall into bed and start something. What is the thing exactly? Why doesn't it last? As Tom (and a pesky narrator) sift through Tom's memories of his time with Summer, the movie clicks onto different days, out of order, letting us eavesdrop on one vignette or conversation or argument after another.

While "(500) Days of Summer" doesn't ask us to adore its rather dislikable protagonist (he's a bit dull -- timid, or arrogant, depending on the moment), screenwriters Scott Neustadter and Michael H. Weber never get too far under Summer's skin. She is primarily a camera object, designed for adoration and self-referential montages shot by Webb in various homages to various film genres. Gordon-Levitt, who resembles the late Heath Ledger a little bit around the face, doesn't bring much of a comic spark to Tom; on the big screen, so far, he has been most effective in dramas, even though he became famous as the youngest alien on "3rd Rock From the Sun."

Nonetheless, the film has an easygoing, inquisitive spirit, heightened by Webb's visual conceits. He was an apt directorial choice, since the script breaks the central relationship into dozens of music-video fragments. Working with "Juno" cinematographer Eric Steelberg ("Juno's" palette was far brighter), he shoots L.A. and some of the downtown area's architectural highlights (Tom, we're told, is a frustrated architect who has mislaid his dreams) with genuine fondness.

There's a cutesy postmodern edge to a lot of the picture; I wish Tom's preternaturally wise younger sister didn't counsel him quite so coyly throughout, and it's disappointing things don't get a little crazier and more disorienting, given the hopscotch structure. But these days it's a relief to see any rom-com that doesn't turn the female lead into a shrew or a harpy or a flat-out witch. Summer is merely cryptic and unknowable, and while that's also a cliche of another kind, Deschanel does some wonderful things to fill in the blanks.