Rob Reiner truly made a name for himself as a director ten years ago with his romantic comedy When Harry Met Sally... The picture remains one of the seminal films in the genre, so perhaps it was natural for Reiner to return to the subject matter a decade later. The twist: whereas Harry/Sally was about a couple coming together, his new movie (called The Story of Us) would be about a couple coming apart. Not a bad idea, especially once big-name stars Bruce Willis and Michelle Pfeiffer signed on. Ingenious as it may seem, The Story of Us is a curious disappointment, a should've-been-great movie that settles for mediocrity.

Willis and Pfeiffer play Ben and Katie, a couple with two wonderful children, a nice house, and jobs they enjoy (he's a comedy writer/aspiring novelist, she designs crossword puzzles). The only thing they don't have is a happy marriage. As the movie begins, they are contemplating a divorce. Ben and Katie put the kids on a bus to summer camp and promptly separate.

From there, The Story of Us does two things. First, it follows the path of the couple as they drift apart and occasionally swing back together. They debate how to tell the kids, she starts dating a local dentist (Tim Matheson), he falls into despair, etc. And, of course, every once in a while they make an attempt to have the old magic come back. The other thing it does is give us flashbacks to different periods in the couple's life that show us how they got to this point. Some of the flashbacks are extended scenes (as in the one where she's too busy listening to the kids argue to talk to him on the phone); other times Reiner gives us rapid montages - brief phrases or actions one after another that follow a certain thread (i.e. Ben and Katie slamming doors after various fights).

What's good about the picture are the performances. Coming on the heels of his work in The Sixth Sense, Bruce Willis is certifiably on a roll. It's nice to see him doing some acting this year instead of just making smirky wisecracks or adopting generic action-guy poses. He gives a solid performance in this film, making Ben goofy but kind of sad. Pfeiffer, I don't need to say, is one of our finest actresses. It's nice to see her embrace her comedic side for a change. Katie, as written, is kind of demanding and shrewish, but Pfeiffer brings some humanity to the part. Both performers put a lot of sincerity into their characters, trying to make them identifiable for the audience.

What's bad (very bad) about The Story of Us is that it takes serious, important emotional issues about marriage and surrounds them with sitcom humor. The screenplay was written by Jessie Nelson and former "Saturday Night Live" scribe Alan Zweibel; the last time Reiner filmed a Zweibel screenplay was the ill-fated North (anyone remember that one?). Although the movie purports to be a realistic exploration of a couple growing apart, there are all kinds of out-of-place gags that seem more suited to a UPN sitcom than a major feature film.

Here's just one example. A therapist tells the couple that there are always six people in their bed: Ben and Katie and each set of their parents. During one major scene, we see them in bed and their parents (Betty White, Tom Posten, Audrey Meadows and Red Buttons) suddenly appear. Ben and Katie are arguing about something that seems relevant to the plot, but their parents (for reasons I won't bother getting into) are singing at the tops of their lungs beside them. I would like to have concentrated on the substance of the fight, but I couldn't because of the distracting attempt at humor going on over top of it. It's a jaw-droppingly bad scene. There are others just like it, including one where Ben gets drunk and throws a tantrum in a restaurant. At another point, Reiner himself shows up playing Ben's friend. He takes the time to launch into a belabored riff on why there is no such thing a person's ass. If the movie wanted to be a broad comedy, it should never have introduced such serious themes. Conversely, if it wanted to take a good hard look at relationships, it should have left out the lame gags (I almost expected to hear a rimshot after some of these jokes).

The Story of Us has enough good elements to make it watchable, but overall I felt let down by it. The main actors are trying to do a good job. They succeed. But the director and screenwriters obviously didn't trust their own material or observations, so they turned the thing into a sitcom. Take this exact same script and replace Willis and Pfeiffer with Patrick Duffy and Suzanne Somers and you'd have a 97-minute episode of "Step by Step."

( 1/2 out of four)

The Story of Us is rated R for language, sexuality, and brief nudity. The running time is 1 hour and 37 minutes.