They are an odd couple as they walk up to their microphones on the opening night of their 2008 tour, at the Palace Theatre in Louisville, Kentucky. Robert Plant, in his first concert since his live reunion with Led Zeppelin in London last year, has seasoned his rock-lord aura with a purple riverboat-dandy vest and white ruffled shirt. Alison Krauss, the most successful singer and fiddler in modern bluegrass, looks like she is on her way to a church social, in a long summer dress, her sharp cheekbones and demure smile framed by a cascade of light-brown hair. But it is clear from their first notes together, the creeping-sigh harmonies of "Rich Woman," a 1955 single by the R&B singer Li'l Millet that serves as the opener on Plant and Krauss' platinum collaboration, Raising Sand: The metal god and bluegrass queen were born to blend.

Backed by a crackling blues-noir band led by guitarist and Raising Sand producer T Bone Burnett, Plant and Krauss reprise nearly all of their 2007 album, gliding in bright parallel on the Everly Brothers' "Gone Gone Gone (Done Moved On)" and comforting each other with pinpoint harmonies on Doc Watson's "Your Long Journey." At times, Plant stalks Krauss' high voice in a ghost-dog croon. Krauss, in turn, shadows Plant on the New Orleans R&B classic "Fortune Teller" with wordless vocal licks, like a prayer call over Radio Timbuktu.

There are Zeppelin songs too, three from the band's untitled fourth album: "When the Levee Breaks," the raunchy "Black Dog" (Krauss sings "Watch your honey drip" with cool glee) and the show's highlight, "The Battle of Evermore," an Arthurian tale recorded by Plant with the British folk singer Sandy Denny. They never performed the song live. But Krauss — who was born in 1971, the year that Zeppelin album came out — harmonizes with Plant like an Appalachian Valkyrie, matching his gritty blues-warrior cries with spearlike notes and church rapture.