At a New York girls' school, a teacher is taunted by her students to tell the tale of the tragic romance that preceded her undeserved notoriety as "Mademoiselle D:" In 1846, Henriette Deluzy-Desportes voyages from England to France to apply for the position of governess in the household of the Duc de Praslin. Once hired, Henriette enters a house that has been poisoned by the insane and neurotic love that the Duchesse bears her husband, the Duc. Finding that her four charges have been emotionally abused and neglected, Henriette works to establish an atmosphere of warmth and love that the children have never known. The Duc, who despises his crazed wife, responds to Henriette's kindness and concern, and an honorable bond of understanding blossoms between them. When this friendship causes gossip, the Duc stops seeing Henriette and effects a facade of reconciliation to placate his jealous wife. He finds that he is unable to foresake his friendship with Henriette, however, and visits her and the children at his family estate in Melan, where they are spending the holidays. After a few fleeting days of happiness, the Duchesse and her wealthy father, the Marechal Sabastiani, appear and dismiss Henriette with the promise of a forthcoming letter of reference. The letter is never sent, however, and Henriette, unable to obtain a job without it, is forced into poverty. When the Duc discovers this, he confronts his vindictive wife and, in an insane rage, murders her. After Henriette is imprisoned for complicity in the murder, the Duc takes his own life rather that implicate her in his crime, and she is released for lack of evidence. Henriette is then befriended by the kindly Henry Martyn Field, a fellow passenger on her earlier voyage to France, who finds her the job at Miss Haines School for girls in New York. Upon hearing her tragic story, the girls award her their friendship and sympathy, and Henriette faces a happier life with her students and Henry at her side.