The tune that the Marquis de Sade keeps humming throughout the film is the French children's song "Au Clair de la Lune" by Jean-Baptiste Lully, the second line of which becomes increasingly relevant - roughly translated, it is "lend me your quill so I can write a word".
The trivia items below may give away important plot points.
Though the events in the film are based on reality, they have definitely been 'tarted up' as the opening disclaimer says. In real life, the Marquis de Sade's second wife, Marie-Constance Quesnet, was living with him at Charenton, along with their daughter. He was sleeping with Madeleine Leclerc, who was only 13. The real Marquis was also extremely obese at that point in his life. His tongue was never cut out, and he died in bed.