British boy band One Direction rose to U.K. stardom on a social-media campaign featuring a super fan and is quickly catching on in America, where One Direction became the first British group whose debut album hit No. 1 in the Billboard albums chart.

The group's performance as the musical act on "Saturday Night Live" this month, a live concert on NBC's "Today" show, sold-out U.S. concerts and cozy chats with Michelle Obama at the Nickelodeon Kids Choice Awards sealed the success of their album "Up All Night."

The band is a product of the U.K. show "The X Factor." Simon Cowell put the five together after they failed as individuals to make it into the final stage of the competition in 2010. One Direction came in third that year, but they have since eclipsed the two acts that beat them on the TV show.

The seeds of One Direction's U.S. success were sown in the U.K. last fall, when an online marketing campaign targeted at European fans quickly spread to both North and South America. Mr. Cowell's Sony-backed record label, Syco, teamed up with Havas-owned London ad agency Archibald Ingall Stretton to create a "transmedia adventure" that engaged fans in both the online and the real worlds.

Over 50 days, the campaign almost doubled traffic to One Direction's website. It attracted 200,000 participants who completed 20 separate challenges, created 12 Twitter trends and racked up more than 2.5 million YouTube views. The idea was not to be overtly commercial but to engage fans and make them feel like part of the campaign.

The campaign's central character was 1DCyberpunk, a mischievous and obsessive "Directioner" (as the group's fans are known) who has stolen the band's laptop and will give it back only if fans prove they are as big a supporter as she is . By the end of the campaign, 1DCyberpunk was as influential in social media as Justin Timberlake and Victoria Beckham, according to the Klout Score metric.

1DCyberpunk issued a series of challenges that helped make sure the band became an active part of fans' daily lives, rewarding faithful followers with an invite to an online album-listening party. The challenges included dressing up as 1DCyberpunk and copying her pink-streaked hairstyle, creating paper dolls of the One Direction boys, quizzes, riddles and sing-alongs. Finally, fans made videos of themselves running to the virtual-listening party for the album.

That party was broadcast to One Direction's online community, so fans could hear the new tracks while chatting to the band and to one another. Clues were given during the event, culminating in a member of the band retrieving the stolen laptop from under a sofa.

Richard Coggin, creative director at London agency AIS, said, "Syco had a lot of great content -- videos, merchandise, singles, albums, lyrics, running orders, signed photos, radio and TV appearances -- and our brief was to glue it all together and engage the fans on a daily basis. Filtered through 1DCyberpunk, the content became more valuable and sought-after."

For the agency, the campaign was a steep learning curve. They created 90% of the content before it started, but they adjusted almost half of it as the whole story unfolded. For example, the first code wheels were cracked in as little as 16 seconds, showing that they needed to test the fans with tougher codes. AIS staff were put on shifts so they could respond round the clock to the global audience. That dedication paid off by creating a U.S. fan base that was desperate to see the band in their home country and to spread the word about One Direction.

The band was involved at every step, dedicating three to five hours a week to creating videos of themselves thanking fans for joining them in the race to retrieve the laptop, discussing their favorite entries and even doing some detective work.