If you're a Sandra Bullock fan and have been waiting for her to be in another good movie, you'll have to keep waiting. "Forces of Nature" isn't it.

Despite a decent first third, this disappointing comedy quickly runs out of steam and becomes surprisingly dramatic — even downbeat — during the final act.

Still, given all her dour performances of late (especially in "Hope Floats" and "Practical Magic"), it's nice to see Bullock smile and be as lively as she is here. Also, she and co-star Ben Affleck have a surprising amount of chemistry (both romantically and comedically).

But they're trapped by a script that goes nowhere . . . so to speak. And as it's constructed, the story can only head in one of two directions (you can probably guess both options). And neither is satisfying.

The film turns the "Planes, Trains and Automobiles" premise on its ear, changing the focus from screwball comedy to romantic comedy.

Blurb writer Ben Holmes (Affleck) is en route to Savannah, Ga., where he's supposed to marry his longtime love, Bridget (Maura Tierney, from TV's "NewsRadio"). Unfortunately, the plane crashes before ever taking off!

So Ben is forced to look for an alternate means of transportation to get from New York to Georgia. Joining him is Sarah Lewis (Bullock), a free-spirited fellow passenger who also needs to get there.

Their attempts are many, including failed efforts by rental car, train and bus. But something strange happens along the way: Sarah tries to get the stiff-necked groom-to-be to loosen up, and her efforts start to work, creating serious sparks between the two.

That — and the fact that Mother Nature seems to be throwing obstacles in their way — creates a dilemma for Ben: Should he run off with his flight companion or complete the trip and settle down with

his more-stable fiancee?

It's a sticky situation, which, unfortunately, screenwriter Marc Lawrence has no idea how to resolve. And director Bronwen Hughes ("Harriet the Spy") has a frenetic style that's completely inappropriate to the material; she has no concept of pacing.

However, it is fun to see Bullock and Affleck interact, even if their characters are as ill-conceived as the script. And Steve Zahn ("You've Got Mail") makes the most of his brief role — he's a hoot as Ben's best man.

"Forces of Nature" is rated PG-13 for profanity (including that R-rated cuss word), vulgar gestures, lewd dancing and sexual discussions, simulated drug use (marijuana), a fistfight and brief, female partial nudity.