Adrian Lyne's Unfaithful is sensational sex-and-its-consequences melodrama. It's a movie in which an ordinary woman from the New York suburbs, played with startling conviction by Diane Lane, begins to sneak away from her beautiful white dream home in the Hudson Valley, deceiving the family she loves in order to indulge in a cathartic sexual fling with a weaselly French hunk who's built like a Michelangelo. Her husband learns of the affair, and when he discovers how far it's gone, his reaction comes as as big a shock to him as it does to us. ''Unfaithful'' is the sort of movie that is generally described as ''steamy,'' which is shorthand for Love Scenes Shot in Cliché Erotic Style of Spice Channel. Lyne has made his share of glorified megaplex porn (''Indecent Proposal,'' ''9 1/2 Weeks''), but he's also the director who, in ''Fatal Attraction,'' struck a raw nerve of infidelity and possession in the age of postfeminist sexual warfare. Far from your typical hazy-filter voyeur, he has an uncanny sensitivity to the hidden jagged byways of feminine desire, and he has staged ''Unfaithful'' with a slow, forbidden-game meditative savvy that allows the audience to linger over each dread-ridden secret and lie.
A loose remake of the 1969 Claude Chabrol film ''La Femme Infidèle,'' this is hardly a movie that's going to be confused with art, but not since the underrated ''What Lies Beneath'' has a domestic thriller been made with so much sultry juice and power. ''Unfaithful'' takes off from an observation that, in its simple way, is quite daring: It says that the deep, rich comforts of family life aren't always enough to keep even a contented person from straying. Connie Sumner (Lane) loves her doting, slightly fuddy-duddy security contractor husband, Edward (Richard Gere), as well as the life they share with their perky 8-year-old son. So why does she let herself be seduced by Paul (Olivier Martinez), the insinuating Gallic book dealer who rescues her from a SoHo windstorm? Is it because he's the sort of philosophe stud who drops pensées like ''There's no such thing as a mistake -- there's what you do, and what you don't do''? The film refuses to offer the usual excuses (e.g., cold-fish husband), and that makes the real reason all the more compelling. Connie slides into a fling because she's a sensual woman whose contentment is tinged with complacency, and because the opportunity presents itself in a way that's too sexy to resist.
Once she gives in to Paul's charms, though, she gets addicted to them. The further he pushes her (a bit of S&M; a shag in a bistro bathroom), the further she wants to go. Lyne shoots the sex scenes like slivers of delirium that flirt with but never slide into fantasy, and Lane, in the most urgent performance of her career, is a revelation. The play of lust, romance, degradation, and guilt on her face is the movie's real story. Her work in the film's intriguing second half, however, wouldn't be possible without Gere's own understated duplicity. As these two circle each other, their marriage held together by the very cycle of forbidden acts that's also tearing it apart, ''Unfaithful,'' like a more languid ''Fatal Attraction,'' becomes a thriller myth of the perils of adultery, which few filmmakers can heighten like Lyne.