A game of opposites seemed to be in motion as Robert Plant and Alison Krauss met onstage last spring at the Louisville (Ky.) Palace to begin a four-month tour.

From stage left came Plant, the earth-shaking vocalist and frontman for Led Zeppelin, the band that reinvented electric blues and scared the daylights out of unknowing parents everywhere beginning in the late 1960s.

From stage right entered Krauss, the most popular and visible face of modern bluegrass, a champion fiddler and a singer with a voice of stirring fragility and plaintive country longing.

With a band behind them fronted by Americana impresario T Bone Burnett, the two launched into the blues and boogie staple "Rich Woman," a tune recast with whispery intensity and an almost swampy musical backdrop.

Throughout the audience, teens and elders were decked out in well-worn Led Zeppelin T-shirts. Such costuming was, perhaps, to be expected. A one-off Zeppelin reunion the previous December had fanned the flames of rumors about a possible full-scale tour by the band. And although the ensuing performance with Krauss would offer four Zeppelin tunes — in understandably revamped form — this was not a night of rock 'n' roll. This was an evening of rootsy excavation, of taking the sounds of early blues, R&B, pop and pre-bluegrass country and forging them into a vibrant new sound.

In a telephone press conference after a wildly well-received performance at Madison Square Garden (fueled by "a somber intensity that they couldn't have possibly summoned before they hit the road," according to a review in The New York Times), Plant, Krauss and Burnett retraced the steps of their roots-music adventure, from their initial meetings to their multiplatinum 2007 album "Raising Sand" to their summer tour.

"It's become quite an illumination, really," Plant said of the traditional inspirations that figure into the music he has been making with Krauss and Burnett and how they differ from the broader blues root of his days with Led Zeppelin. "The qualities and inherent elements of a lot of the music that has been exposed to me, they're very familiar. There is a lot of blue in whatever this music is. And bearing in mind that Alison comes from a bluegrass root, what has been created with the chemistry between the three of us has its own kind of genre, really.

"I hear it in the hill music of Kentucky I've been exposed to in the last two years. I'm not talking about contemporary bluegrass. I'm talking about mountain music. Hill music. So I find there are so many familiarities to me that I don't feel at all estranged from my old root."

Krauss fondly recalled her initial meeting with Plant, prompted by a phone invitation from Plant to collaborate, that helped set the stage for "Raising Sand."

"We were rehearsing in an Armenian dance hall that looked like it hadn't been touched in 40 years," Krauss said. "I walked into the room and saw this big pile of hair. I walked his way and he turned around. He had his glasses on and said, 'Ah, there you are.'

"We talked about Ralph Stanley and traveling through the Appalachian Mountains and how much we loved traditional music. And I thought, 'What an interesting person.' "

Burnett — who turned the Americana roots-savvy soundtrack to the Coen brothers' film "O Brother, Where Art Thou?" into a multiple-Grammy-winning crossover hit in 2000 — produced "Raising Sand." He was especially impressed with the way Plant and Krauss worked outside their vocal comfort zones in the studio and again on tour. He gives especially high marks to Plant for his singing on "Don't Knock," a Staples Singers spiritual that has been added as an encore during recent concerts.

"I'm completely surprised by Robert's singing on 'Don't Knock,' " Burnett said. "In particular, to hear him singing those gospel blue notes, those free-type things. He's not imitating anybody. They're just coming right out of him. And then I'm surprised by hearing Alison singing as hard as she is on some of the tunes. That's thrilling to hear."

Plant, Krauss and Burnett aren't the only ones onstage reveling in the rich, rootsy music. Singer-songwriter Sharon Little was supplementing her songwriting income with work as a waitress at the beginning of 2008. Then she was picked by Plant and Krauss as the opening act for their entire tour — a trek that was well under way before her major-label debut album, "Perfect Time for a Breakdown," was released.

"What they create sounds like a Gypsy world," Little said by phone before a concert with Plant and Krauss last month in San Diego. "It fascinates me. I'm totally mesmerized by it. To me, Alison Krauss sounds like an angel. Her voice and my voice are two worlds apart. She hits these notes I can't even imagine hitting. It's also exciting for me to see a musician like Robert Plant really get down and dig into his music. And T Bone just seems to wrap up what they do together.

"I'm well aware no one comes out to the shows — well, maybe a few ... like, perhaps, four — to see me. But I still feel like a spoiled kid. I've been given the audience of Robert Plant, Alison Krauss and T Bone Burnett to play to. So, yeah, it's a really, really wild thing. I thank them onstage every night."

Only once in the press conference did a question referencing the continually rumored Zeppelin reunions come up. Plant bypassed the topic completely and asserted that the deceptively low-tech music he is making with Krauss is as difficult — and, ultimately, as rewarding — as any artistic endeavor he has undertaken.

"Singing in this revue is not easy," Plant said. "In fact, it's the most challenging event that I actually remember. It's because I'm working with other voices all the time. I'm learning so much about America, Americana and American music from Alison, T Bone and the whole band (which includes multi-instrumentalist/ singers Buddy Miller and Stuart Duncan, bassist Dennis Crouch and drummer Jay Bellerose).

"My whole deal about singing is that I don't just go into remote control to satisfy my ego. I go into a place I can actually look forward to, no matter how tired I am. I'm a very fortunate man to be in this environment, to be learning every day.

"I stand on the side of the stage when I'm not involved, sometimes pinching myself, saying, 'Am I really in the middle of all this?' I couldn't wish for anything better."