WHO Old-timey folk-rock act formed by singer Marcus Mumford and three guys who aren't his sons. It's already huge in Australia, where the band unseated Susan Boyle on the charts. An L.A. show attracted Jake Gyllenhaal and Benicio Del Toro.
SOUND For an acoustic act, Mumford and Sons shows are surprisingly rowdy, Appalachia-goes-Arcade Fire affairs. "I always thought acoustic music was chill — but I was so wrong," says organist Ben Lovett. "Blue-grass is the best dance music ever."
BACKSTORY The band got its start at Bosun's Locker, a tiny club under a London Cornish pasty shop. "It was only big enough to stand and drink one beer," says Lovett, laughing.
BOOKWORM Mumford runs an online book club (first read: All the Pretty Horses) and says he's inspired by mythology. "The Odyssey is as much folk music as "House of the Rising Sun': It's specific to a time but lasts for generations," he says. "I'll never sing, 'I went to buy a Diet Coke.' I want my songs to last forever."
KEY TRACK When Mumford played "Little Lion Man" on Letterman, that night's guest Ewan McGregor requested tickets to their show.
This story is from the April 1st, 2010 issue of Rolling Stone.