In the 18th Century, just before the French revolution, multiple religious cults of the dark side were engaged in secret maneuvers under the name of Revolution. "Le Chevalier DEon" (Knight DEon) a diplomat and a special agent working for Louis XIV, is fighting against those cults, and also on a mission to find the killer of his own sister. A mysterious cult organization called "Rose Cross" stands in his way, using vicious powers to create a dark creature, "Gargoyle" to take over Europe.

Le Chevalier dEon, Charles de Beaumont, was a real-life diplomat and spy in 18th century Europe, most famous for being of questionable gender. DEon worked as a spy, politician and soldier for King Louis XV for many years, before returning to France during the rule of King Louis XVI. After his return to France he lived and dressed as a woman. It was not until his death that doctors confirmed that he was a man.