Allegro traces the checkered career of concert violinist Danny Sachs. We see his steady progress as a promising young student, adored and encouraged by many. He and Ruth, a talented young pianist who becomes his accompanist, fall in love. Danny wins the International Violin Competition in Moscow and makes a brilliant debut in Carnegie Hall. His career is off to a soaring start.

Amid the glamour and stress of his success, Danny becomes increasingly discontent. He leaves Ruth and begins a dangerous affair with the wife of his wealthy patron, delving deeper into the world of cocaine. He falls desperately in love with Natalya, a renowned ballerina, only to be thwarted. His drug abuse worsens and his erratic behavior lands him in jail. His career seems ruined.

Slowly and painfully, through rehabilitation, he recovers from his addiction and his troubled past. He finds his way back to his concert career and back to Ruth, seeing her in a new light.