"Forty-Second Street" is a giant step toward every producer's dream: a new musical with absolutely no new ingredients.

The story -- it's the one about the Broadway chorus girl who replaces the leading lady at the last possible minute -- hails from a novel and movie of the same name, and has been only nominally tampered with by co-librettists Michael Stewart and Mark Bramble. The songs -- including "Lullaby of Broadway," "We're in the Money" and "You're Getting to Be a Habit With Me" -- were written by the old Hollywood team of Al Dubin and Harry Warren for an assortment of 1930s movies, such as "Forty-Second Street" and "Gold Diggers of 1933." The dance numbers carry the signature of Gower Champion, but it is Champion paying tribute to Bushy Berkeley and the extravagant movie musicals of his day.

The dancing is emphatically the best reason to see this show, which opened last night at the Kennedy Center Opera House, but it may not be enough of a reason for anyone afflicted with the old-fashioned idea that a musical ought to have characters one can care about and a story that ties the songs together and lends them meaning. In "Forty-Second Street," the characters have a rude habit of changing character without explanation, and plot developments fly in and out with the suddenness of meteorites.

It's not that the story is, on its own hokey terms, illogical, but it is ridiculously abrupt. When director Julian Marsh (Jerry Orbach) rushes to the Philadelphia train station to tell chorus girl Peggy Sawyer (Wanda Richert) that she's the new leading lady, we expect her to be surprised -- after all, he fired her only about a half-hour ago. But we are not at all prepared to hear her say, "I'm sorry, Mr. Marsh, show business isn't for me. pI'm going back to Allentown." This is a completely new sentiment, born on the spot for no good reason except to give him cause to sing the seductive "Lullaby of Broadway."

That number turns out to be a real theater-shaker, with half the company descending the train-station steps to join in wooing her back. Here, as elsewhere, the philosophy at work in "Forty-Second Street" seems to run close to that expressed in one of its lyrics: "Who cares if there's a plot or not, when you've got a lot of dames?" This show has a lot of dames -- a cast of 60 in which they figure very prominently -- and a lot of production gimmicks, too. One song, "Shadow Waltz," makes Tammy Grimes a dancing silhouette behind a vast sheet. "We're in the Money" has virtually the whole company (and, most conspicuously, lead dancer Lee Roy Reams) tapping across a stageful of huge coins. Another number is set in a revolving health salon, and still another is performed in wheelchairs.

"Forty-Second Street" is reputed to be an unusually expensive musical, even as musicals go these days. The figure of $2 million has been mentioned, and whatever the real number, no one can accuse producer David Merrick of squandering his money on invisible assets. While most musicals have been getting smaller and smaller, Merrick has put more people on a stage than any producer in memory -- maybe that's his way of getting our attention after a five-year absence from the theater. And he has given his vast cast enough scenery and costumes to house and clothe a small Third-World nation.

One could wish that Merrick and Champion had put their money, and their considerable talents, to some more vital purpose than trying to recreate a piece of 1930s kitsch. Or, failing that, one could with they had simply done it better. The original movie was, and remains, far more affecting. But "Forty-Second Street" will surely get sharper as the weeks go by, and even now there are pleasures to be grateful for.

As the bitchy, over-the-hill star, Tammy Grimes is extremely funny when given anything to be funny with -- and the rest of the time she has that amazing voice of hers, like creme de menthe poured over dry ice. As the genius director, Jerry Orbach is a spelendid confusion of energy and sourness fast words and slouched shoulders. And true to the spirit of the show-within-the-show, the younger performers without star billing are the real stars of "Forty-Second Street": Wanda Richert, a dynamite dancer and singer; Reams, who dances up a storm at the smallest provocation; and all those hot-stepping chorus girls and boys.