At first glance, Connie Sumner (Diane Lane) would seem to have it all. She has been happily married for eleven years to Edward (Richard Gere), has a lovely eight-year-old son (Erik Per Sullivan), and a spacious country home just outside of New York City. On a particularly windy afternoon in the Big Apple, Connie is literally blown to the ground and on top of used books dealer Paul Martel (Olivier Martinez), an alluringly handsome and animalistic Lothario. Invited up to his apartment to treat her cut knee, nothing happens between them until Connie finds that she can't get him out of her mind. Despite knowing better and experiencing great confusion as to why she is drawn to this younger man, Connie is unable to resist starting an adulterous fling. A sea of increasingly convoluted lies to husband Edward results leaving him immediately suspicious, as he hires a personal investigator to follow her tracks. As one of Connie's experienced friends, Tracy (Kate Burton), tells her, "Affairs always end up badly."

Perhaps the most humanistically genuine motion picture Adrian Lyne has yet put to celluloid, "Unfaithful" takes an unflinching and emotionally rattling look at the recklessness of infidelity and how it can destroy the lives of all three parties involved, leaving no one satisfied. While thriller elements are introduced into the story over an hour into the proceedings, Lyne resists the temptation of resorting to horror movie cliches like he was guilty of in "Fatal Attraction." Here, Lyne's intentions are set on a notably higher and more thought-provoking wrung. This may, at least in part, be due to the film's origins. It is based on Claude Chabrol's 1969 French drama, "La Femme infidele," and has been adapted into an unusually astute American screenplay, by Alvin Sargent (1999's "Anywhere But Here") and William Broyles Jr. (2000's "Cast Away"). The sumptuous cinematography, by Peter Biziou (1998's "The Truman Show"), is thoroughly evocative and rich.

While Diane Lane (2001's "Hardball" and "The Glass House") has been in movies since the young age of 12, she has rarely gotten the credit, or quality of roles, that she deserves. For the first time in her underrated career, Lane has been blessed with a front-and-center, three-dimensional stunner of a role that the best actresses in Hollywood deserve to be envious of. As the confused and guilt-stricken Connie, Lane is asked to run the full spectrum of emotions, from unexpected joy to emptiness to heartbreak, and her every step is a flawless grace note. If a stronger female character is brought to the screen this year, it would come as a real surprise.

In a subtly affecting part that gradually becomes wrenching, Richard Gere (2002's "The Mothman Prophecies") hasn't been this good since 1994's "Intersection," which, incidentally, was also about a love triangle that ends in tragedy. As Edward becomes more and more aware of his wife's deceitfulness, the anger, hatred, and insecurities that come with it are palpably felt by Gere's powerhouse turn.

Spanish heartthrob Olivier Martinez (2000's "Before Night Falls") is the kind of stunning example of masculinity that Lane might even believably choose over Gere, and he does a fine job even when his thick accent is difficult to understand. Meanwhile, young Erik Per Sullivan (TV's "Malcolm in the Middle" and 2001's "Joe Dirt") is masterfully understated as Connie and Edward's precocious son, particularly for someone of his age.

If the first 85% of "Unfaithful" doesn't, or rarely, steps wrong, director Lyne runs into trouble while searching for a workable finale. The last fifteen minutes feel like a collection of unsteady scenes, each one setting itself up for a possible ending until it moves onto the next one. When the film does culminate in its final conclusion, the results leave you thinking, to be sure, but is so ambiguous that it proves an unsatisfying last chapter in a remarkably assured and touching picture. At the same time, at least it doesn't resort into a "Friday the 13th"-style slasher flick, a 'la "Fatal Attraction."

While the ultimate ending could have used another rewrite, everything else about "Unfaithful" is worth celebrating. It is not often in today's times of flashy special effects and mindless action set-pieces that viewers are gifted with a rare adult film that presents a serious view of sex, love, and relationships. As a showcase for some marvelous actors still at the top of their game, and a poignant portrait of a family in the midst of unraveling, "Unfaithful" gets it just right.