No one, of course, will ever get to be Cary Grant again. That happened only once. But what, to be frivolously hypothetical, if it had never happened at all? If there'd never been a Cary Grant, would we have noticed a certain sad deficiency in the world? Would it have always seemed a trifle incomplete?

Cary Grant was brilliantly versatile and played many kinds of roles, yet at the sound of his name, we picture something grandly self-assured and glamorous. Cary Grant often came across as a luxury item, yet when you look back on his career, you realize he was absolutely indispensable -- to the movies, to Hollywood, to popular mythmaking and to each of us individually.

He was the least expendable extravagance in the world, an elegant Gibraltar.

"Cary Grant: A Class Apart," a new 90-minute documentary at 8 tonight on the Turner Classic Movies channel, is quite the lush and deluxe item itself, appreciative of Grant's grace and talent in fresh and refreshing ways in the centennial year of the actor's birth. Shame on the producers for allowing such cliches as "the press had a field day" and "the camera loved him." But then the camera did love Cary Grant, didn't it? Loved him madly. Loved him as it loved few others of the male sex.

You and I may try hard to imagine getting up in the morning and looking in the mirror and seeing Cary Grant there, and what a kick and a thrill that would be, but in his public personal life, the kind big movie stars have to lead, Grant was unimpressed by that vision in the mirror. No matter how makeup artists and photographers and eyebrow-pluckers dolled him up, he never came across as a preener. Maybe that's part of what made him not only so attractive to women but so basically unthreatening to men.

There was his matter-of-factness about being gorgeous, and there was the extreme of that gorgeousness. Cary Grant was a class apart, like the documentary's title says -- a class so far apart that only a fool would aspire to it.

In terms of structure and technique, there's nothing all that extraordinary about Robert Trachtenberg's documentary, but when the subject is Cary Grant, you don't want to drown the lily, and the filmmakers don't. They assembled a cast of experts on cinema -- critics and film historians and such, who analyze Grant's work. They also rounded up an impressive roster of people who knew him personally, including ex-wives Barbara Grant and Betsy Drake.

With obliging candor, Drake confirms that Grant did indeed experiment with LSD, as rumors have long alleged, but only after she used it first as an aid to psychoanalysis. Another long-standing rumor about Grant is faced straight on: Did he have homosexual affairs as well as affairs with women, and were he and Randolph Scott lovers during a period when they shared a house back in the '30s? Drake is consulted again. She didn't pay attention to rumors about Grant's sex life, she says, because "we were too busy [copulating]." Then she speculates that maybe -- maybe -- he was bisexual, but it's pointed out in the script that Grant and Scott were so amused by rumors of their having been boyfriends that they made jokes about it. Scott even autographed a menu from a memorable dinner party: "To my spouse, Cary -- Randy."

If they were confident enough to pull such stunts as that, for better or worse or richer or poorer, it seems very doubtful that Cary Grant and Randolph Scott were candidates for a same-sex marriage.

As often happens in documentaries like this one, some of the experts rounded up for the chore become so scholarly about the subject that they threaten to take the fun out of considering it. A couple of such presences are on display in "Cary Grant," but on the bright side, there's Elvis Mitchell, a film critic with a gift for taking contagious delight in performers, writers, directors, whatever. He finds disarmingly original ways of appreciating Grant; by the time Grant made "Charade," Mitchell says, "he's pure mythology," perhaps 100 percent illusion.

The clips are well-chosen and nicely rationed out over the course of the program; it's an intelligent mix of reminiscence and rediscovery. Grant made more than 70 films in his career, but the number seems so much less when one looks back. There were popular and critical successes, of course; critical successes that the public wasn't buying; and, here and there, a few all-out bombs.

Oddly or not, Grant was never nominated for an Oscar for playing a variation on the familiar, blithe Grant persona, as if that somehow lacked importance; instead Grant was nominated for atypical roles like the hard-luck average Joe he played in "Penny Serenade." Insanely, of one of the century's great dream casts -- Grant, Katharine Hepburn and James Stewart in "The Philadelphia Story" -- Grant was the only one not nominated for an acting Oscar.

The documentary is full of little-known or half-forgotten lore about Grant: He and Douglas Fairbanks Jr. decided which roles they would play in "Gunga Din" by flipping a coin; Grant himself suggested the inspired torn-dress routine that led to a hilarious walk across a nightclub floor with Hepburn in "Bringing Up Baby"; Paramount wanted Stewart to star in Alfred Hitchcock's "To Catch a Thief," but Hitch wanted Grant and lured him out of temporary retirement to play the part -- and play it as no one else, including Stewart, could have.

There are rare sights, including color footage shot on the set of "Gunga Din" and an 8mm comic western that Grant made with ex-wife Barbara Hutton's son, Lance -- and words from an autobiographical essay that Grant wrote, reluctant though he was to talk about himself and thus threaten the myth he had come to represent.

Years ago, I was promised an interview with Grant during a Washington stopover of his, but it fell through and was reduced to a mere phone conversation. I asked Grant why he hadn't written an autobiography and he said that when he was gone, other people who'd known him would write their own books with their own allegations and that he couldn't fight those posthumously anyway. I got to shake his hand once at the American Film Institute a few days after an essay I'd written about him was printed in The Post. As he shook my hand he said, "You were too kind," and I said, imagining it to be clever, "How could anyone be too kind to Cary Grant?"

The comment didn't do him justice, the essay certainly hadn't done him justice, and excellent though it is, the documentary probably doesn't do Grant justice, either. But it does launch a month-long, Tuesday-night festival of Grant films on TCM -- the best place to see the best movies ever made. In addition, Warner Home Video has just released a collection of Grant titles, though none is a classic.

One, however, is a foolish personal favorite -- a mediocre movie that Grant makes shine like gold. It's called "Night and Day" and is a fanciful version of the life and career of Cole Porter. Pauline Kael wrote once that men as divergent as Lucky Luciano and John F. Kennedy thought Grant should play them in the movies. It was an honor that came to few and that none deserved, but how Porter must have adored it.

The trouble with reliving Cary Grant's life via a book or a documentary like TCM's is that one must relive his death, too. Documentaries cannot be given artificial happy endings. But Grant earned the immortality that true screen icons get, and if it could be said that there are degrees of immortality, which it really can't, then Cary Grant has to be one of the most immortal guys who ever stepped in front of a lens.

And back to that question, what if he hadn't? Well, who knows? The whole universe might have been thrown out of whack, the polar ice caps might be melting even faster, or something ghastly like that. At the least, one can say without much fear of derision, the fabulous collective dreams that movies can inspire would inarguably and lamentably lack for luster.

Thanks, God, for making such a good Cary Grant.

Self-assured and glamorous but never a preener: Grant (in "North by Northwest") is the focus of a TCM documentary.Star quality that made him attractive to women and yet never threatening to men: Cary Grant with, from left, Grace Kelly in "To Catch a Thief," Katharine Hepburn in "Bringing Up Baby" and Ingrid Bergman in "Indiscreet."Grant with Hepburn and James Stewart in "The Philadelphia Story." He was the only one of the three not nominated for an Oscar for the film.