Gwyn, the young advertising writer in David Frankel's whip-smart romantic comedy "Miami Rhapsody," is a peculiar sort of modern movie heroine. Though most of the characters in the film are glib and sophisticated and casually decadent in the amoral fashion of the spoiled bourgeoisie, Gwyn (Sarah Jessica Parker) is a moralist trapped in the body of a Victoria's Secret model.

Like many women her age, Gwyn is beginning to think it's about time she settled down, got married and had a family. And so when her boyfriend, Matt (Gil Bellows), pops the question -- "I think you ought to marry me" is the way he puts it -- she immediately says yes (even though he offers only a napkin ring to commemorate the event).

Almost as soon as Gwyn consents, though, she discovers that all the happily married people around her are having affairs and behaving like utter brats. At her sister's wedding, for example, her mother (Mia Farrow) informs her that she is having a passionate fling with Antonio (the enormously appealing Antonio Banderas), the much younger nurse who takes care of Gwyn's invalid grandmother (Mary Chernoff). Then, after some well-placed comments from her mom, she learns that her father (Paul Mazursky) has been getting it on with his travel agent for years. And that her older brother, Jordan (Kevin Pollack), has abandoned his pregnant wife, Terri (Barbara Garrick), to shack up with a fashion model (Naomi Campbell). By the time she walks in on Leslie (Carla Gugino) -- yes, the little sister who just got married -- in bed with an old friend, Gwyn is so freaked out about making a commitment that she's on the verge of meltdown.

By having Farrow and Mazursky play Gywn's mother and father, Frankel is acknowledging his influences right up front -- as well as setting a very high mark for himself to reach. Like Mazursky's "Down and Out in Beverly Hills" and Woody Allen's "Hannah and Her Sisters," "Miami Rhapsody" is a satire on the foibles and rationalizations of the morally-filthy rich; like those earlier films, it is also brainy, funny and flawlessly performed.

Unfortunately, Frankel may have borrowed too much for his own good. As his characters quip and bicker with one another, muddling through their doubts and entanglements, you immediately recognize the directorial voice. Unfortunately, it's not Frankel's voice, but the echo of others. In "Miami Rhapsody," Frankel acts as a sort of impressionist, skillfully reproducing the styles and mannerisms of the filmmakers who have worked this turf before him.

In most cases this lack of originality would be disastrous, but surprisingly, it doesn't intrude much on our enjoyment -- mainly because Frankel has shown such exquisite taste in mentors. And in leading ladies too. In the past, Parker has been a Valley-girl Rapunzel, letting down her golden locks for the likes of Steve Martin and Nicolas Cage. In those movies she was ditsy, but here there's a quick-firing intelligence inside that pretty noggin. To her, all this rutting and cheating is disgusting, and because whatever she's thinking jumps right out of her mouth, she can't help but let everyone hear about it.

The challenge for Frankel here is to make Gwyn seem headstrong and principled without having her come off as a prig. That Gwyn's conversation is peppered with quips and one-liners that Parker delivers with the dead-on accuracy of a sharpshooter doesn't hurt. Also, the presence of Mazursky and Farrow helps enormously to soften Gwyn's youthful bite -- and to disguise the fact that at times she talks like a character out of a bad Neil Simon play. These seasoned pros contribute something more than suavity and style, which they have in buckets; they give the film a sense of reality and perspective. Gwyn may be frazzled about the marital chaos around her, but they simply take it in stride, confident that long-term relationships are full of ups and downs.

The mellow assurance of these actors provides Frankel's directorial debut with a depth of experience it desperately needs; when they're on the screen, the movie seems to slip into a more soulful, human mode. And in those moments -- plus some others as well -- "Miami Rhapsody" deserves to stand alongside the movies that inspired it. Miami Rhapsody, at area theaters, is rated R. CAPTION: 'Miami' Nice: Mia Farrow and Sarah Jessica Parker in David Frankel's smart romantic comedy that honors its film influences.