Taylor Swift was somewhere over the Pacific Ocean, on a flight bound for Tokyo, when one of her managers walked over to her seat. "He said, 'I don't want you to panic,'" Swift recalls. "And I said, 'The song leaked, didn't it?'" Swift was right. "Mine," the lead single from her upcoming album, Speak Now, was all over the Internet 12 days before its planned release date. "We'd been on superhigh alert," says Scott Borchetta, president of Swift's label, Big Machine. "Once it hit Perez Hilton, I called everyone and said, 'We've gotta go now!"

Within a few hours, Borchetta's team had sent the single to radio and orchestrated a rush-release to iTunes. "Plans changed," says Borchetta. "That's the way of the world."

By the time Swift landed, "Mine" was Number One on iTunes (the song sold 297,000 copies in four days). "I turned on my phone and there were texts saying, 'Congratulations!' " recalls Swift. "A leak is so out of my comfort zone, but it ended up good in the end. It made me so emotional that I started crying."

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"Mine" follows in the tradition of Swift's biggest hits; it's a catchy country-pop love song that's ripped straight from her diary. "I was reflecting back on a boy I liked at a certain time," she says. "The song is about what it would be like if I actually let my guard down." The tune also raises expectations for the singer's third album, which retailers expect to sell around 800,000 copies its first week, making it one of the biggest records of the year. (Her last album, 2008's Fearless, is six times platinum.)

At home in Nashville, Swift is bracing for a whirlwind. "I'm obsessed with being busy," she says. "It's hectic and nonstop, but I'm so ready."

This is a story from the September 2, 2010 issue of Rolling Stone.

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