At the exact mid-point of the two-hour “Unfaithful,” the husband finds out his wife has been cheating on him. It is easy, then, to divide the movie into halves: the good hour and the not-so-good hour. “Unfaithful” starts smart and then chickens out.

Initially, it is about a well-off suburban housewife named Connie Sumner (Diane Lane) who stumbles into an affair with a 28-year-old French casanova named Paul (Olivier Martinez). She did not seek it out, but when the opportunity presents itself, she finds herself taking advantage of it.

The first sexual encounter is shown to us via her flashbacks, giving the sense that she, too, was merely an observer, and not a participant. ‘This isn’t something I would do,’ she seems to think during the train ride home. ‘I’m perfectly happy in my marriage.’

That is one of the film’s smarter elements: Her husband, Edward (Richard Gere), is good. Their marriage is comfortable — but not complacent — and they have a young son (Erik Per Sullivan) whom they adore. There is no reason for Connie to have an affair. Even she can’t figure it out. If she had unsatisfied yearnings, they were unknown even to her.

At first, this may seem like a flaw in the screenplay. Characters should not, generally speaking, have unmotivated actions. But I contend this is more realistic than the standard movie plot of “my husband’s a jerk, so I cheated on him.” (Not to say that doesn’t happen in real life, too.) Connie’s senseless addiction to Paul makes more sense, in its way, than many cinema adultery plots do.

Lane’s performance is brave and excellent. She endures a lot, and her character is compelling. It takes skill to keep an audience’s interest when the action is more emotional than physical, and Lane (directed by Adrian Lyne) pulls it off admirably.

Her co-star’s half of the movie does not fare as well. That is when Edward learns of the affair and acts upon his knowledge, and the film reverts to standard Hollywood claptrap. Interesting how adultery is made to appear immoral and devastating, but other crimes — I won’t spoil anything for you — are apparently OK in the world of movie people.

There, finally, is the film’s major mistake. Screenwriters Alvin Sargent and William Broyles Jr. have spent considerable time establishing the destructive nature of Connie’s relationship with Paul, only to undermine all that work with a turn of events that is neither believable nor, even by the movie’s own standards, morally correct. Don’t bother being impressed by the film espousing old-fashioned, marriage-is-sacred values, because it doesn’t last beyond the first hour.

You can hear the filmmakers’ sigh of relief: “We just came dangerously close to making an original, thought-provoking film! Thank goodness we swerved to avoid it! Whew!” “Unfaithful” cheats on itself and retreats to comfortable — and complacent — territory. Too bad.

C+ (; R, scattered harsh profanity, some very strong sexuality, one scene of strong violence.)