(FORTUNE Magazine) – From 1987 to 1997, MasterCard maxed out five advertising campaigns--and failed to narrow the gap with Visa. So when the company decided to get a new ad agency, it looked like desperation. To McCann Erickson, it looked like opportunity.

McCann assigned a core creative team of three--Joyce King Thomas, Jeroen Bours, and Jonathan Cranin--to prepare a pitch. The trio, who had been working together for two years, conferred with the strategy team and brainstormed intensively for a month. "We were very comfortable working together, so we debated everything freely," says Thomas, now McCann's chief creative officer in New York City.

The breakthrough came to Cranin in the shower: the tag line "some things money can't buy" to anchor the ad. Back at the office, Thomas caught the spark and began crafting a spot around it. Inspiration struck two weeks later, as Thomas and Bours batted around ideas over coffee and bagels on a Sunday morning.The first ad would be set at a baseball game, feature a list of ordinary transactions, and lead to the setup: "Priceless." Recalls Thomas: "We knew we had it."

MasterCard agreed, even after a different spot tested better in research. "Intuitively, we knew the insights made it more than just another ad," says chief marketing officer Larry Flanagan, then head of U.S. advertising. Gut feeling proved right. Since 1997, MasterCard has added new U.S. credit cards at more than twice Visa's rate. And the award-winning campaign's versatile format and simple appeal have also made it a global winner: Spots have been tweaked for audiences in 105 countries and 48 languages.