For almost any other live radio show, it would have been disaster: The musicians arrived, the instruments arrived, the star--Garrison Keillor--arrived. But the music did not.

The cast of "A Prairie Home Companion," in town for tonight's sold-out show at Wolf Trap Center for the Performing Arts, had taken a different flight than their suitcases full of sheet music.

But this is a veteran crew--this being the last of the season's 34 shows--so they commence to writing new charts on blank paper and laptops in the sweltering rehearsal hall at Wolf Trap yesterday afternoon.

After a few minutes, Keillor shuffles in, wearing a black suit and white T-shirt, Birkenstocks and dark shades. He moves in slow motion, greeting the musicians with a light shoulder touch or a gentle tap atop the head.

The only apparent problem is finding the register of his singing voice, as the band launches into "Turn the Radio On."

"I don't have a bass voice," he says, finding his pitch. "Today, I've got a baritone."

Then, like a sensible, broken-in Mackinaw, the other singers slip around Keillor's baritone, and rehearsal kicks off.

It has been 25 years since Keillor's first "Prairie Home Companion" was broadcast live before 12 audience members in an auditorium at Macalester College in St. Paul, Minn. Since then the show has become a franchise, turning Minnesota Public Radio into a powerhouse. For listeners the show is something substantive to hear on weekends, two days traditionally given over to radio dreck.

More significantly, the show has crowned Keillor--in the words of writer Dave Barry, who knows a thing or two about comedy--"the great American humorist." Granted, Keillor occupies a fairly narrow range of American humor--the nonprofane, satiric extended monologue--but he is the current king of his genre.

"Public radio has allowed [Keillor] to become a national figure of the kind we haven't had since Mark Twain or Will Rogers," says Steve Behrens, editor of Current magazine, which covers public broadcasting. "TV doesn't let individuals have very much time onstage. They don't let monologuists on TV very much."

The tall, bushy-browed native of Anoka, Minn., refuses to celebrate his 25-year run--"I don't know if we're going to say five-and-a-half words about the anniversary," says Keillor, 56. "There just isn't a single sensible word to be said about it. If you stay around long enough, there you are."

Tonight's show marks the end of the season. "A Prairie Home Companion" resumes live in October. Keillor will take the time in-between to finish a screenplay adaptation of "Wobegon Boy," his 1997 novel.

Each weekend he takes two leisurely hours to spin tales of his mythical home town of Lake Wobegon, Minn., where--according to Keillor's trademark phrase--"all the women are strong, the men are good-looking and the children are above average." His show appears on more than 460 stations, with an estimated audience of 2.7 million. Locally it is heard live on WETA-FM (90.9) Saturdays at 6 p.m. and on tape Sundays at 10 a.m.

Keillor's monologues are bittersweet paeans to a nonspecific fictional time and place that many listeners have grabbed onto as their own. His show is radio's "Rashomon"--small-town nostalgists nestle in and hear the warmth and homeyness; urban ironists smirk knowingly and hear the detachment and satire.

Keillor maintains homes in Wisconsin and New York. But he has visited Washington only twice in seven years, which is somewhat surprising given that this area is one of public radio's more fertile markets. Maybe it's because he has little good to say about Washington: He bears a well-chronicled enmity toward the political right and dislikes one of the city's chief industries--punditry.

"Washington has the disadvantage of being an invented city in that it is very often out of touch with reality, which it wouldn't be if the capital were in a major city," he says. "The impeachment travail is the prime example of that in our time. I don't think anybody in the business in Washington can appreciate how surreal these talking heads appeared on Sunday morning, going on for months--a year--talking about the same thing."

The Republican hard right has come under Keillor's criticism in the past. It's likely because they are alien to both of his worlds: the Lutheran upper Midwest and the liberal Upper West Side.

But his show here promises to be patriotic, not political. Even if it weren't passe, the audience likely would not hear Lewinsky jokes from Keillor, who is an admirer of the Clintons.

Tonight's show will sample American music--Duke Ellington, Fats Waller, gospel and country--and offer a survey of American history--he will talk about the Constitution and Declaration of Independence. Keillor has built a show that celebrates Americans' pursuit of happiness thanks to the "value of indifference."

"John Adams wrote that one-third of the people supported revolution, one-third were loyalists and the rest were indifferent to its outcome," he says. "If we'd been a nation of heroes, we never would have lasted because then, having raised the sword for that cause, people would have been ready to raise it for the next cause. Hamilton and the Federalists might have raised an army to fight Jefferson, and every time an issue came up, people would be dying in the streets for it."

Instead, thankfully, "the indifference to politics is a great human quality," Keillor says. "It's the wish to enjoy the pursuit of happiness and enjoy life itself, just to scratch along, and go to the dance, and enjoy going out to dinner."

CAPTION: "If you stay around long enough, there you are": Garrison Keillor rehearsing for tonight's show.

CAPTION: A not-so-dressy rehearsal, above, with, from left, Garrison Keillor, Linda Williams, Kate MacKenzie and Robin Williams. Below, Rich Dworsky accompanies.