It's a story as old as showbiz itself: The star falters. A kid takes her place, sings her heart out...and a new star is born. But when the showbiz fable unfolds in the high-toned precincts of La Scala, with enough Sturm und Drang to suit Wagner, well, that's why the opera world is still agog over Kallen Esperian's debut.

Last May, Esperian, a 28-year-old newcomer from Memphis, was understudy to soprano Katia Ricciarelli at Milan's legendary opera house. Ricciarelli, 43, a world-class soprano, was playing the title role in Verdi's Luisa Miller. "She wasn't singing real well," recalls Esperian. "She was having problems."

Especially with the audience. Italian opera buffs, as vocal as they are passionate, booed Ricciarelli mercilessly. In fact, they booed her second performance to a dead stop. When it resumed, the furious singer dropped to her knees, raised her arms and swore at her tormentors, "May God send a damnation on you."

That left Esperian to go on with the next show. "I don't think I've ever been so nervous," she says, remembering the catcalls rained upon Ricciarelli. "I had never heard anything like that. I thought, 'Please, God, let them be sympathetic to a newcomer.' "

They were more than sympathetic. Once they heard Esperian's soaring, lyric soprano, they were ecstatic. "Their reaction surpassed my wildest dreams," she says. From the first-act curtain calls, the La Scala crowd never stopped cheering. The critics were similarly wowed. "A star is born," gushed La Notte. "She conquered the audience," wrote the authoritative Corriere delta Sera.

A professional singer for only 3½ years, Esperian grew up near Chicago and graduated in 1983 from the University of Illinois with a degree in vocal performance. Married during her senior year to Tom Machen, a voice teacher, she moved with him to Memphis where, in 1984, she won the Metropolitan Opera's Mid-South Regional Auditions. The following year she became a top winner of the prestigious Opera Company of Philadelphia-Luciano Pavarotti International Voice Competition. Since then, she has become Pavarotti's protégéé.

With highly acclaimed debuts at both Carnegie Hall and the Metropolitan Opera over the winter, this has been Esperian's most successful season yet, but it was her spectacular premiere at La Scala, says Esperian, that "made people sit up and take notice." It also taught her a more difficult lesson. "To be a singer, you not only need to have the voice," she says, "you have to have the nerve."