"I'm in Miami, b****!" That's Niall Horan, nineteen, screaming to a crowd of 15,500 at the American Airlines Arena in downtown Miami in his best American accent. The catchphrase would be repeated by all five boys of One Direction throughout their 72 hours in the Magic City, a whirlwind three days of back-to-back shows, a (top-secret) music video shoot, and a strategically booked day off: Saturday in the city where the heat is on.

After playing 60 sold-out shows in sixteen countries on their Take Me Home World Tour, this is the guys' first time soaking in the tropical concrete jungle where bigger, louder, sexier, and hotter is better, and they're excited. To top it off, the lads are playing their two-hour set in the be-all, end-all stadium of American sports, the home of LeBron James and the NBA championship–winning Miami Heat. (Fact: The team gifted customized black jerseys to the band when they arrived. Baller.)

The weather is perfect but hot when I check into The Ritz-Carlton Key Biscayne, Miami (1D's home base for the weekend), six hours before showtime. There are already clusters of teenage girls loitering near the hedges leading up to the property, camped out in the restaurants, and riding up and down in the elevators. By the pool, taking shade under lush palm trees, are more girls, eyeing two fit bodies in the shallow end: Niall and Harry Styles, nineteen. Sunbathing, laughing, and chicken fighting, they'd pass for any other guest—if not for being stalked and ogled by every girl in the place.

Three hours later, I'm lost backstage in a blur of larger-than-life basketball decals and blue 1D crew signs plastering the walls. Inside a door marked ONE DIRECTION: WARDROBE, I see a Union Jack tank top being steamed (Liam Payne, nineteen, would later wear this to open the show). ONE DIRECTION: MUSICIANS ... keep walking. ONE DIRECTION: CATERING.... Finally I know I'm in the right spot when I see six burly, sunburned bodyguards. The sign on the door reads ONE DIRECTION: ONE DIRECTION.

There are a few things you should know about 1D, if you don't already. After losing season seven of the U.K.'s The X Factor, the band defied all odds to become the most successful reality-bred group in the world—and yet they prefer T.G.I. Friday's or Chili's to any Michelin-starred grub. They were all raised by working-class families in England and Ireland, and before their rise to fame, Harry spent Saturdays mopping floors at the W. Mandeville bakery in his hometown; Louis Tomlinson, 21, flipped burgers at the Doncaster Rovers football stadium; and Zayn Malik, 20, was a waiter at his family's restaurant (his first plane ride wasn't until the band hit it big). Nowadays you can't go anywhere without seeing signs of their multimillion-dollar endorsement empire: a fragrance, a school-supply line at Office Depot, a bedding collection, a HarperCollins book deal, a Nabisco partnership, and, in true pop-star form, Hasbro dolls. They spent more than half a year shooting their new documentary, This Is Us, directed by Oscar-nominated filmmaker Morgan Spurlock of Super Size Me fame. And now they're in Miami and ready to live it up.

"This is the most sunburned I've ever been in my life!" Niall tells me, explaining that he wanted to enjoy the pool with Harry. "I'm not going to be a prisoner in my room—and the fans weren't intrusive." Some artists at their level of fame seem to expect fans everywhere, but not Niall: "I don't see how you could get used to people screaming in your face, and anyone who says different is lying." Zayn sits down next to me; his brown eyes sparkle. "We're humbled that so many fans have bothered," he says. The most soft-spoken of the bunch, he's quick to tell me he was not by the pool today—he can't actually swim. "It was a bit too busy for me," he adds. "I like to chill somewhere quiet. I get pleasure from the little things in life." Suddenly there's a new voice in the mix: "Great to see you, darling." I turn around to find a statuesque Louis standing over me. "Are you all right?" he asks in typical Brit fashion.

Next I'm being whisked off to catering— these guys are hungry. And, whoa, can they eat. A chef from London is making chicken for Harry and Zayn. A pint of Ben & Jerry's is thawing on the table. "Do you want some bananas Foster?" Harry asks in an American accent.

I've spent more time with the band this spring than with my own family, and this chilled-out backstage atmosphere seems to be the norm. They're not treated like untouchable rock stars, certainly not by anyone in their 50-person crew, comprising mostly no-nonsense Brits. "We haven't actually got a rider," Liam says of the contracts most stars create to make sure their dressing rooms are stocked to their liking. "We never actually sat there and made one. They bring us the stuff they think we like." I must look surprised, because he adds: "We're not really that diva."