Olivier Rousteing, the creative director of Balmain, is too busy to be a bad boy.

The enfant respectable of Paris fashion (who was just 25 when he took over the couturier in 2011) was in New York for barely 48 hours last week, to introduce his new BBuzz Bag at Saks Fifth Avenue.

Wearing a scoop-neck top that showed off his signature pectoral cleft, the young designer presided over a sedate cocktail party Friday night at Le Chalet on the store’s ninth floor. “Leaving tomorrow, arrived yesterday,” he said, with no trace of weariness. “I have to go back to Paris to work on my men’s wear prefall collection and my women’s wear.”

He was also preparing for the release of a documentary about his life, “Wonder Boy,” which debuts this week on French premium cable, ahead of a theatrical release.

“It is about looking for my biological parents and my origin,” said Mr. Rousteing, who was adopted as an infant by a couple in Bordeaux, France. Cameras filmed him 24 hours a day for two years, he said. “But I got used to it.”