The main character is Van Renbork, a fallen noble that, together with the elder sister, was abused throughout his youth for the pleasure of the royal consorts of Estoul, a 1700's France-like kingdom.
It's only after causing the horrible suicide of Van's sister that the royal family decides to definitely shut Van into the palace dungeons, under the (true and proved) charge of conspiracy against the Crown.
Ryouki's story begins many years later, after the death of the royal consorts, when a truly unusual visitor goes to see Van: Eldia, the late king's arrogant and snobbish elder daughter. Plotting against her own little sister for the succession to the throne, Eldia reaches an evil agreement with Van, freeing him.
But Eldia (and many others, too!) will be VERY sorry for this foolishness, because Van, driven from a lifetime of hatred, can now implement his own plans, turning the royal palace into a den of iniquity and causing the ruin of the kingdom.
[From a forum post by Baldo]