“This is the story of boy meets girl.” So begins the bittersweet odyssey of greeting-card writer Tom Hansen, the love-starved twenty-something in (500) Days of Summer who harbors dreams of becoming an architect and romancing puckish co-worker Summer Finn.

As Tom, Joseph Gordon-Levitt signals his moods in easy-to-read body language. When his heart is aflutter, as it so often is when Summer rewards his attentions, his eyes sparkle and he walks with a spring in his step; at one point, overcome with bliss, he’s moved to dance. The scene is deliberately over the top, suggesting a winking homage to Ferris Bueller’s memorable lip-sync of “Twist and Shout” (Tom busts his moves to Hall & Oates’ “You Make My Dreams”), but it works. Even when performing for the camera and stepping outside the story, Gordon-Levitt seems totally genuine, and his joy lights up the screen.

Gordon-Levitt, who starred in the middling NBC sitcom 3rd Rock from the Sun at age 15 and has since proved himself a capable lead in independent dramas like Brick (2006) and The Lookout (2007), carries Scott Neustadter and Michael Weber’s story skillfully. He has a winning presence – when his shoulders slump, our spirits sink with them. You root for Tom to get the girl not because she’s right for him – she’s not – but because the alternative is too painful for him to consider.

It is Tom, not the maddeningly aloof Summer (Zooey Deschanel, of Yes Man), who serves as the movie’s emotional anchor. This is a boy-meets-girl fable told from the boy’s perspective, and Tom, the naïve romantic who self-consciously stammers through their first encounter, seems destined for heartbreak. He’s unguarded and generous with his affections in a way Summer is not.

If Tom recalls a more career-oriented Lloyd Dobler, Summer is no Diane Court. She’s the kind of character Deschanel, with her impish smile and deceptively innocent big blue eyes, plays so well: flighty, quirky and full of surprises that can delight as easily as devastate. To Tom, she begins as a tantalizing impossibility – she’s out of his reach, or so he thinks. Then they bond over the anguished music of the Smiths – an ominous choice, perhaps, in a movie that reflects emotional sea changes with a mood-sensitive soundtrack. But it’s a start.

From there, (500) Days of Summer moves indirectly toward a conclusion spelled out at the onset. We know Tom and Summer will part – it’s the next step in the classic boy-meets-girl scenario, and besides, the movie’s narrator tells us as much – but we don’t know how or why.

Clues emerge early on. Tom wants a relationship. Summer wants to keep things casual. Tom believes in love, convinced he can pry his way into Summer’s heart if only she allows him an opening. Summer thinks love is a myth, and confounds Tom with mixed signals. It’s not that Summer is cruel, though she can be thoughtless. It’s simply a case of two people looking for different things, and Tom’s unwillingness to accept that.

There are euphoric highs and crushing lows in Tom’s quest to keep Summer in his life, some of them expected – a post-breakup meltdown at work seems almost obligatory – and some of them less so. Marc Webb, making his feature-length debut after directing music videos for Green Day and 3 Doors Down, keeps the story grounded enough to give such moments the feel of true romance – hilariously in the best of times, heartbreakingly in the worst.

Sandra Bullock recently complained about most romantic comedies, “They’re not funny, they’re not romantic, and they’re not written well for women.” It’s a fair point, but (500) Days of Summer is an exception. It is smartly written and refreshingly affecting on its own terms. At the very least, it will make you smile.