MARK TWAIN TONIGHT, conceived by and starring Hal Holbrook; at the Kennedy Center Opera House through Saturday.

Hal Holbrook has been doing his Mark Twain act off and on since 1954, and it's on again, at the Kennedy Center, through Saturday. Whatever you think of one-man shows, whatever you think of impersonations, erase it all from your mind. "Mark Twain Tonight" is to the usual run of these entertainments what King Kong is to the usual run of monkeys.

Holbrook hasn't actually become Twain, but he has reached a level of such commanding spontaneity that it nearly amounts to the same thing. Once he gets going, you stop thinking about Holbrook at all. You start thinking about Twain -- about his rangy brilliance, his unsettling humor and his extraordinary ability to see and make others see the contradictions, evils and pretenses of the society that produced him (although he could be swept up in that society's tides and fashions as much as anyone).

Holbrook's impersonation begins with a magnificent makeup job and a flowing white wig any bird or rodent would be happy to claim for a nest. But it is the voice that carries the performance -- a voice that sounds, simply, the way Twain's prose reads. It sounds like the voice of a 70-year-old, cigar-smoking, Missouri-born genius who has been a lot of places and seen a lot of things.

Holbrook is performing in the 2,300-seat Opera House -- a large setting for a production whose entire physical inventory consists of an actor, a lectern, a table, a chair, a pitcher of water and two stacks of old-looking books. But Twain, who devoted many months of his life to the lecture circuit, was used to halls this size. He was used to filling them with people and laughter, in a day when ordinary Americans would pay to hear extraordinary Americans talk. So the grandeur of the auditorium, in this case, only enhances the conviction of the show.

The title suggests a billboard for one of Twain's lectures, but Holbrook makes shrewd use of material that wasn't published or made public until many years after the author's death -- particularly his "Letters From the Earth," in which the archangel Satan reports to the Creator on the strange creatures called humans and their strange religion called Christianity.

Satan inquires into, among other things, the business of Noah and the Ark, wondering how this bowl-shaped, rudderless, clearly incompetent piece of nautical engineering could possibly accommodate all the millions of species on the planet. The explanation, according to Satan, is that Noah had to call it quits long before he had everyone aboard -- when a messenger came to report a sudden approach of dinosaurs and other "horrible vast masses of tumultuous flesh . . . struggling, fighting, scrambling, screeching, snorting. . . Coming to get saved from the flood. And not coming in pairs, they were all coming: they did not know the passengers were restricted to pairs . . . and wouldn't care a rap for the regulations, anyway -- they would sail in that Ark or know the reason why."

Satan also comments on the curious fact that men and women "prize copulation above all other pleasures combined," yet they have devised a heaven in which it doesn't exist. Praying, singing and playing the harp are what people do in heaven, Satan reports.

This was the kind of material that Twain withheld at the behest of his wife and his literary adviser, William Dean Howells. But Holbrook's inclusion of such passages helps convey the delicate line Twain often trod in his public appearances, anxious to speak his irrepressible mind on the one hand and to preserve his standing as America's white-haired, white-suited literary eminence on the other.

Perhaps the most amazing thing about Holbrook's performance is that he doesn't quote verbatim. He edits and improvises and combines. And he doesn't do the same routines every night. Yet it all sounds like Twain. Perhaps he fumbles and shuffles and struts around the stage more than necessary, and perhaps he puts too much of the professional actor's skill into his readings of various parts from "Huckleberry Finn," and perhaps he takes excessive pleasure in facing away from the audience rather than toward it. But next to the sum of what he has accomplished, such criticisms add up to nothing at all.