It is show-biz's favorite myth -- usually confined to fairy tales and B movies. On the eve of an important opening night, the show's star -- a temperamental prima donna -- is unable to perform.

Her talented understudy -- an unassuming, fresh-faced young thing -- is plucked from the chorus, dashed through rehearsals, thrust on stage to thunderous applause and emerges a star.

On Wednesday night, this storybook dream came true for 18-year-old Susan Jaffe, a native of Bethesda who was to have made her debut in the corps de ballet of the American Ballet Theatre during its current four-week engagement at the Kennedy Center Opera House.

Instead she stepped into the toe shoes of principal dancer Gelsey Kirkland, who was dismissed along with Patrick Bissell for "gross breach of contract" in a surprise announcement Tuesday.

"Somewhere in the back of my mind I thought it [getting the opportunity] might happen," Jaffe said yesterday, alternately chain-smoking Merits and chain-chewing orange bubble gum amid the bustle of the corps de ballet dressing room. "But I talked myself out of it. It seemed too farfetched."

Dressed in an oversized gold sweater -- "a castoff from my uncle" -- and pink tights, her thick brunette braid pinned close to her head and her large cat-green eyes shining, Jaffe recalled the events leading to her debut with principal dancer Alexander Godunov in the ABT premiere of "Pas d'Esclave."

"It was about 2 p.m. Tuesday and Gelsey hadn't shown up. I was walking down the hall to rehearsal -- I'm an understudy in 'Push Come to Shove' -- and Charles France [Abt Artistic Director Mikhail Baryshnikov's assistant] pulled me aside and said I was to go on.

"I thought I'd just do the rehearsal," She said, pausing mid-mascara to daintily remove a chewed piece of gum from her mouth and insert a fresh one. "But after the rehearsal they told me I'd be going on that night. I didn't think I'd go on for quite a while . . . maybe in a matinee much later.

"I was shocked, excited, stunned. My first gut reaction was that I wanted more rehearsal. I'd only rehearsed with Sasha [Godunov] maybe four or five times.

"Of course I wanted it, but I was just very scared.Everyone says that first performance is the worst.

"After you learn the stage it becomes your home, and the audience visitors who come to see you. I thought I'd be doing corps work, with maybe a solo later on. But to start with a pas de deux . . ."

Jaffe rushed to telephone her father, an engineer still living in Bethesda.

"I said, 'Dad, you'll never guess what happened,'" she said. "He was so excited for me he screamed into the phone."

She had been "so sure" she wouldn't appear opening night that she hadn't gotten him a ticket -- but managed to scrounge up one for the standing-room-only performance. Although she didn't have time to call her former ballet teachers from the Maryland School of Ballet, they heard "through the grapevine" and were also in the audience.

Looking more like a sweet schoolgirl than the exotic harem slave she portrays in the pas de deux, Jaffe seems solely occupied by dance. Her manner is unpretentious and warm, her youth underscored just briefly when someone switched on a radio blaring "Saturday Night Fever" and she began to move her feet to the disco beat.

One "walk-through" rehearsal and one full costume rehearsal later, and she was in the wings waiting for her cue. "I don't remember a thing about it," said Jaffe, who won glowing praise in reviews. "Just that I went on, tried to love the part and do my very best."

She does, however, remember the three curtain calls and two bouquets she received. "We hadn't rehearsed a curtain call at all, so we just improvised," she said. "I remember thinking I had always been clapping for the people on stage, and now I was out there. It really felt neat.

"I'm glad I get to dance it again. Now that my first performance is over, I'll feel more at ease and let go. I want to interpret the part more. But the time you're pleased with your performance is the time you should quit ballet."

Jaffe, who has been dancing since she was 8 left Walter Johnson High School at age 16 to join the Ballet Repertory Company -- the second company of the American Ballet Theatre -- almost didn't make it into the ABT.

"I auditioned in June, but they didn't take me until August," she said, squirming slightly. "It's very embarrassing. You see, I was overweight."

By dance standards, 110 pounds was too heavy for her nearly 5-foot-5 frame. While she was working to shed pounds -- "a diet doctor who didn't work, a week of fasting and then healthy eating" -- she received an offer from the Berlin Ballet.

"It was a corps contract, plus solos," she said, "and would have been about a third more than I'm making here [$300 a week]. But I've always wanted to dance with ABT. I wanted to work under the direction of Misha. I think he's incredible, and I could probably learn a lot."

Now, she said, tightening the ribbons on her toe shoe, "I know I made the right choice. I honestly haven't had time to think about what's happened. I don't think it would be good to really think about it too much. It might be too overwhelming."

Even the one potential note of sadness didn't darken Jaffe's dream debut. Her mother, who first encouraged her to take up dancing, died this summer.

"But I know she was there," Jaffe said, unblinking. "I felt her. My mother wouldn't have missed my first performance."