To give a brief, bare-bones summary, Marie Du Gard was engaged to Sebastien de Saint Vallier's ether-inhaling brother, Nardi. She left Nardi at the altar in her single act of disobediance, and also her autocratic father in a sense, had a brief coupling with Sebastien, and went to America. Three years later, she returned with a trunk full of film and cameras, intended to be (in the manner of the Lumiere brothers) a filmmaker. In the turn-of-century Paris, this is unheard of, and the rest of the story develops the lingering attraction and budding romance between Sebastien and Marie (despite their differences.)
"Sebastien knew Marie Du Gard slightly better than her father realized... It had been a fleeting, feverish encounter on a rainy August afternoon that had made no sense then and made even less now. He remembered that afternoon three years ago as a kind of a blurred, hysterical dot on the continuum of his otherwise orderly life, a little moment hat was easier to pretend had never happened than to explain in the context of his normally sound, exemplary conduct."