Beauty and the Beast

"They're both soulmen," Burnett raves the next day on a tour bus parked next to the Palace. "There is gospel in that song — the 'Prince of Peace' — and the same darkness you find in bluegrass and murder ballads. It is a darkness that is absolutely in Robert, in his voice and life, that Alison understands." Later, Plant admits he went onstage that night "laden with nerves. I wanted to make it work for her. Alison was stepping out of her world." So is Plant, who turns 60 in August. "I'm listening intently up there, because I've never heard another singer alongside me.

"And white mountain music — it's new to me," confesses Plant, who has a deeper knowledge of American blues. He cites Kentucky singer Roscoe Holcomb's chilling treatment of "House of the Rising Sun." "It's a black song, but the way he deals with it — he is invoking spirits that are now driving me to some other place."

Krauss has led her own band, Union Station, for almost 20 years. But singing with Plant "is like standing on a cliff," she says over the phone, a few shows into the U.K. leg of their tour. "In bluegrass, the lead vocal stays consistent. Everything around it is the harmony. But Robert is always improvising, and I gotta watch him. I said, 'If you're gonna go up, just look up, so I can see where you're going.' "

Even that doesn't always work. "We had something the other night," Krauss says, laughing. "We ended 'When the Levee Breaks,' and I thought, 'What just happened?' We were all over the place. Robert just shrugged."

"I only work on impulse," Plant says cheerfully in his defense. "There is nothing I feel I can't do. And I'm wholehearted about this," he says of their partnership.

"This is not about a move for me," Plant insists. "This is a genuine shifting of space and air."

Read the entire interview in the new issue of Rolling Stone, on stands June 13, 2008.