Not often is a theater the star of a show. But that's the case with "Radio City Music Hall's Grand Re-Opening Gala," a slightly spectacular tribute to the mammoth Manhattan showplace that recently emerged from a $70 million restoration.

A cultural as well as architectural landmark, the art deco behemoth no longer shows movies, as it did for five decades or so after its opening in 1932. The era of the "movie palace" ended with the rise of television and, later, those suburban multiplex match boxes. Ironically, the trend now, even in the mall cinemas, is back to bigness.

Mercifully, meanwhile, the Music Hall survived, mainly by playing host to stage shows, special events, pop-star concerts and TV specials. Like this one. Only, usually, better.

A radically truncated version of a charity gala taped Oct. 4, the "Gala" is a two-hour show crammed into a one-hour time slot, at 10 on Channel 4. Over the years, the Music Hall stage served as stomping ground for some of the greatest talents around. This was not to be one of those nights. The lineup is less than luminous, with singer Mary J. Blige displaying a new extreme in cleavage (which the director tries hard not to show) but singing hum-drummily; hunky popsters 98 Degrees doling out dull lulls in contagiously sleepy style, and poor old Lou Bega trotting "Mambo No. 5" around the track yet one more--and please God, last--time.

At least Bega's tune, while no "Vida Loca," lends itself to a splashy production number. The other songs just sit there, with the performers doing the musical equivalent of whittling. Tony Bennett was on the bill at the gala in October, but he's nowhere to be seen in the TV version, apparently having taken up residence on the cutting room floor.

Reliable, buoyant Billy Crystal amusingly demonstrates the amazing elevators that are built into the Music Hall stage. An entire orchestra can rise up from seemingly subterranean depths, then appear to float above the stage. As part of the demonstration, Crystal does a slight sight gag involving the outfit he's wearing. It's cute and gets a big laugh from the crowd in the hall. But lamebrain producers give the joke away in the opening teaser, thus ruining it for viewers.

Not surprisingly, one of the producers is Pierre Cossette, prominent among Hollywood's tackiest hacks. At nearly every juncture, the approach is unimaginative and dispiriting, and for all the hypey hoopla about restoring the place, little of the Music Hall other than its stage is shown. For other details, you have to tune in to a second special, "Radio City: The Story Behind the Showplace," narrated by Lauren Bacall and premiering Tuesday night at 8 on cable's American Movie Classics.

Many splendid TV events have been staged at the Music Hall. Mary Martin hosted an Easter special there in the '50s. David Letterman did one of his "Late Night" anniversary shows there, back in the '80s when he was still funny. Most monumental of all was Alexander H. Cohen's "Night of 100 Stars," which aired on ABC once, in 1982, and has never been seen again. Rising up to stage level on the elevator that night was, unforgettably, James Cagney, who burst into tears at the roaring ovation he received. Wow, what a memory.

Absolutely none of this is recalled in the restoration gala. It isn't gala, really, or worthy of its subject. But any chance to see the Rockettes is worth it, and they fortunately both open and close the hour. The finale also includes Liza Minnelli, wobbliness personified but still possessed of a certain fizzy pizzazz.

She's undergone a $70 million restoration too, hasn't she? Well she's still in there pitching, the Music Hall looks glamorous, and it's just a pity the special turned out to be such a humble, bumbling bust.

CAPTION: The Rockettes are kickin', but the rest of NBC's "Radio City Music Hall Re-Opening Gala" certainly isn't.

CAPTION: Liza Minnelli, one of the few not offering up Z's in NBC's Radio City special.