Two options.

On one hand, "The West Side Waltz," the new comedy by Ernest Thompson that opened a three-week run last night in the Kennedy Center Opera House, can be viewed as a pretext for contemplating Katharine Hepburn. Since Hepburn is one of the more amazing septuagenarians on the planet, she should probably be gazed at admiringly, as long as she is willing to suffer gazers.

On the other hand, "The West Side Waltz" can be viewed as a play about some of the lonely, eccentric denizens of a dowdy apartment building in New York, among them a haughty, fiercely independent septuagenarian named Margaret Mary Elderdice. Judgments get stickier here.

Margaret Mary Elderdice may be an interesting creature, too. It is hard to tell, since that is the Hepburn role and from the start her special presence rules the stage like a benevolent autocrat. The actress doesn't play the character as much as she invades it. Thompson may have set out to write a gentle comedy, but he has ended up with a vehicle.

As drivers go, there is probably none better than Hepburn, who fulfills all the mandates of her mega-stardom with considerable flair. She remains a heartily handsome woman, seemingly above the petty erosions of time. Her hair may be graying, but the way she styles it, or doesn't style it, makes an irrefutable case for unkempt chic. The celebrated cheekbones continue to rival Mount Rushmore as one of the country's more durable works of sculpture. And even though Thompson's script calls for her increasing immobilization, Hepburn's vigor gives a lie to the cane, the walker and finally the wheelchair she is required to employ.

She can take a perfectly negligible line--say, "You can show your wife a good time in Pittsburgh, if that's not being too contradictory"--and by giving it that indomitable Hepburn inflection make it sound acutely clever. Jokes about pooper-scoopers are pretty much the bottom of the line. Hepburn, however, talks about a "poopah-scoopah," and the aristocratic tone virtually guarantees a laugh. Elizabeth Taylor's fabled persona took her only partially through "The Little Foxes," and then gave out. Hepburn's carries her triumphantly over the finish line with energy to spare.

For most theatergoers that will be more than enough. There is, however, the attendant business of Margaret Mary Elderdice and the sad creatures who inhabit the fringes of her life. One of them, Cara Varnum (Dorothy Loudon), a fiftyish spinster of limited intellect, would like nothing better than to weasle her way into Margaret Mary's apartment and take care of her. Margaret Mary would like nothing less.

Just so long as she can make her way to the piano and play the waltzes that put her "in a dreamy" state, Margaret Mary is content. Regular as clockwork, however, Cara drops in for a chat, a bonbon or two and the expectation of hearing a kind (or at least not a disparaging) word. Silly as a goose and abject as a disgraced courtier, Cara has one redeeming feature: a minor skill on the violin. And for Margaret Mary, an afternoon duet is more satisfying than a solo.

What Thompson is trying to trace in the evening's six scenes is the humbling of Margaret Mary, who comes to extend a little charity toward the human race. The play's psychology, however, is not abundantly clear. Having resolutely resisted Cara's sugary advances, Margaret Mary promptly turns around and offers her quietly decaying lodgings to a would-be actress from Brooklyn, who's more or less adrift in the world since she discovered her husband is a homosexual. As Regina Baff plays her, the 30-year-old boarder is utterly charmless and a tad demented. Why does Margaret Mary bother? Does this sourpuss really remind her of herself as a young woman? Hard to swallow.

Several scenes later, the improbable guest finds a young lawyer (Don Howard) to her liking and announces that she is moving out. Margaret Mary is deeply injured by such a display of initiative. The blow, we are given to understand, knocks her off her pedestal and paves the way for Cara to move in. Much of this is very odd stuff, indeed. "The West Side Waltz," if not exactly hack work, is in no way so adroit a play as Thompson's previous hit, "On Golden Pond."

Hepburn's performance skirts the difficulties with unflappable majesty: She accentuates the caprice in Margaret Mary's strange decisions and since audiences tend to grant her any caprice whatsoever, she is out of the woods. When, at long last, she asks Cara to settle in, the invitation passes for the gesture of a magnanimous woman. But if caprice and magnanimity are what we expect of Katharine Hepburn, they may not be the right answers for Margaret Mary. Somewhere along the line, the search for some honest dramatic solutions has gotten short-circuited.

Not having Hepburn's pre-ordained persona to rely on, the other actors must fall back on material that is only fitfully helpful. As the Romanian super who sees communists behind every steam pipe, David Margulies contributes a flagrantly caricatural performance, wrenched from the sitcoms. Don Howard's appeal is limited by such perfunctory lines as, "I'm a lawyer. Would I lie to you?" Loudon, however, gives a game portrait of the harmless spinster, for whom life's big plunge is running for the board of the tenants association. She may be a human doormat asking to be tramped on, but Loudon ferrets out her buoyancy and optimism.

Still, you are left with the impression that these people don't belong to the same faded world. Thompson's play really hums as a play only when Hepburn and Loudon team up for one of their duets--Hepburn, pounding the piano with an aplomb that dismisses all the wrong notes as if they were disobedient subjects; Loudon, scratching away furiously at the violin, the fear of lagging behind concentrating her features like a tub of old pudding. "Now we're cooking," croaks Hepburn, and indeed, for a moment they are.

Otherwise, "The West Side Waltz" functions primarily as a shrine. Hepburn will not disappoint the faithful and even the unconverted may have trouble resisting her commanding manner. The play will appeal mainly to armchair sleuths and amateur psychologists: in short, those who, pondering inconclusive evidence, enjoy imagining what might have been.

THE WEST SIDE WALTZ. By Ernest Thompson. Directed by Noel Willman; set, Ben Edwards; lighting, Thomas Skelton; costumes, Jane Greenwood. With Katharine Hepburn, Dorothy Loudon, David Margulies, Regina Baff, Don Howard. At the Kennedy Center Opera House through April 10.