"Platinum," the musical that opened Saturday at the Kennedy Center Opera House, has dazzling advantages, flashy novelties and the ultimate in technical equipment. But it badly lacks a sense of direction.

The plot idea is not unpromising. It involves Lila, the still glamorous, leggy film star, and Dan, a rock meteorite aware that he's over the hill at 30. Their October-July attachment shatters Dan's relationship with Crystal, a squirt battling for her own shot at the top of the charts.

As Lila, Alexis Smith (looking, she says, like Angela Lansbury after years of "Mama") gives a socko performance of a '40 movie star determined to revive her career by recording a disco hit in '78.

Another advantage for "Platinum," which takes its title from the term for a million-selling record, is the orchestrations Jimmie Haskell and Fred Thaler have provided for Gary William Friedman's score. These sounds expand on that amplified mode Burt Bacharach used as such an innovation in "Promises, Promises."

And the setting is novel, based by designer David Hays on "the newest environmental recording studio in Hollywood," a maze of sound and mixed-media gizmos, complete with a steaming Jacuzai.

The intent, I suspect, is to forge a musical, generational link between the romanticism of the '40s and the energy of contemporary disco. The symbolic struggle, however, gets unstuck because, try as "Platinum" does to be blaringly mod, its heart is in Lila's period.

This is not surprising, because TV's endless reruns of old movies have captured youthful hearts with their charming dialogue, sura and personalities. By presenting their rock star as a fan of old movies, authors Will Holt and Bruce Vilanch bring both heart and interest to the character.

But they can't disguise the limitations of rock or the superficiality of life and death on the record charts - here, today, nowhere tomorrow. Or is that their point?

The finale, "Old Times, Good Times," reflects this unresolved conflict. To come up with a hit, Lila goes back to her tap-dancing of a '41 flick ("we even danced on airplane wings"). She proceeds to teach the tap dance to everyone in the studio, and all wind up in white as though they've filched more than the finale costumes from "A Chorus Line."

What is one to conclude from this number, for which a climax is almost purposefully avoided? It ends not with a bang but a whimper. Its visual vitality couldn't, of course, be reflected on a recording, so what does that say about Lila's comeback? Thrilling as it might be to lead the charts for a week, would "Old Times, Good Times" make it?

With the issue joined between then and now, "Platinum seems to prefer the past to the present though it's not bold enough to say so.

There's a lot of material to be jettisoned if "Platinum" is to survive its month-long shakedown. With enceptions, such as Lila's introductory "Back With a Bent" and her and Dan's "Movie Star Mansion" and " [WORD ILLEGIBLE] the numbers are overlong. In real life Lila might make the same mistake of stalking out at a critical point three times, but dramatically that's both a plot contrivance and a bore.

For all its flash and energy, "Platinum" drags and waffles. Also directing, choreographer Joe Layton is footlose in the staging.

But Damita Jo Freeman, Robin Grean and Avery Sommers form an all-purpose girly trio, with Freeman especially striking. Stanley Kamel's recording mastermind suggests what the performers must think of their managers and Tony Shultz symbolizes classical students who turn to rock.

A compelling actor, Richard Cox plays Dan with an honesty that makes his Lila liaison plausible and he sticks to realism for his nude duck in the Jacuzzi. As the unsympathetic Crystal, Lisa Mordente, Chita Rivera's daughter, as the thankless task of playing a cheap second-rater.

Star Smith, whose highlights include a misty scene from an imaginary movie, dazzles brightly indeed in a part that furthers that "I'm Still Here" number Yvonne de Carlo sang in Smith's Tony-winning "Follies." Like the character she plays, Smith was a star of the '40s - and she doesn't look any 57, either.

Herrole could do, however, with that very zest her character scorns from her years of playing Mame and Dolly in the dinner theaters. Lila's a tad too selfish and bitchy to wind up in the same league with those life-loving dames.