The Marquise Eugenia di Maqueda, an orphan raised by the nuns, marries Raimondo Corrao, but on their wedding night she finds out that he is her brother. The piece of news is in a letter written from Paris by their father, a womanizer who lives and hides from them in the French head town. The pair decide, to avoid the scandal, to live as brother and sister. Publicly, they take a vow of abstinence to "achieve sainthood" instead of consummating their marriage. He will later leave for the war in Libya, she will find solace and sexual satisfaction in the arms of the family chauffeur.