In the early '40s, you went to movie theaters to see the top live entertainers—five, six shows a day, and, oh yeah, the movie in between. In December 1942, at the Paramount in Manhattan's Times Square, headliner Benny Goodman introduced an "added attraction": 27-year-old Frank Sinatra, out on his own after a stint with Tommy Dorsey and his band. The teenage girls had been saving up their shrieks for hours. "What the hell was that?" said Goodman, stunned by the tsunami of noise. Just the sound of a pop-music revolution—and a greeting for America's original teen idol.

Sinatra set kids on fire the way no singer ever had before. He came on as one of them, fresh out of Hoboken, N.J. There was no macho swagger—yet. (That came later, along with the hints of mob connections, spectacularly public infidelities and the deeper vocal artistry.) For now, he was boyish, vulnerable and so alarmingly skinny (5'10", 137 lbs.) that the girls scarcely knew which he needed more, love or a square meal.

Sinatra was a ballad singer, smooth and dreamy. He spun visions of romance and sentiment that soothed the wartime jitters. "There was great loneliness, and I was the boy in every corner drugstore—the boy who'd gone off, drafted, to war," said Sinatra, who was 4-F due to a punctured eardrum.

Yet this boy also had something more—pure sex appeal. "I love all those girls the same as they love me," said Sinatra. And for as long as each song lasted, he made them believe it.