This is going to be a weird one, folks. Animated by Production I.G., it's beautifully crafted. Filled with great sword fights, beautiful settings, perfect music...and one of the weirdest stories I've seen. Le Chevalier D'Eon is loosely based on the real life of d'Eon de Beaumont, a knight and spy for Louis XV, king of France. There are plenty of other historical characters like Louis XV, Marquise de Pompadour, Comte de St. Germain, Francis Dashwood, the Russian Empress Elizabeth, and even Maximilien Robespierre. They are mixed in with purely fictional main characters Robin, Durand, and Teillagory.
D'Eon, Robin, Durand and Teillagory all work for King Louis XV as spies in his personal spy network le Secret du Roi. When the story opens, D'Eon is mourning the death of his sister, Lia, a member of le Secret du Roi who died while on duty in Russia. In history, D'Eon did not have a sister named Lia, but D'Eon himself spent a good portion of his life as a woman and went by the name Lia. In the anime, instead of using the real D'Eon's transgendered nature, he is occasionally possessed by the vengeful spirit of his dead sister. Even Lia is unsure of who murdered her and why, and her soul will not rest in peace until the mystery is solved. D'Eon serves as a vessel for her soul because of their close bond as siblings.
When Lia's body appears floating inside a casket on the Seine, it sets D'Eon off on a journey that will forever change his life. On the top of her casket is the word "Psalms" written in blood, and it is the only clue he has to follow. With Queen Marie's help he discovers that this is somehow related to the Royal Psalms and the Revolutionary Brethren, a group of Poets attempting to decipher the Royal Psalms (a book that denotes those chosen by God to be King). Louis the XV conscripts D'Eon into le Secret du Roi along with the Queen's page Robin, fellow le Secret du Roi member Durand, and former Musketeer Teillagory, and sends the four of them off to find the missing Royal Psalms. D'Eon believes this will lead him to the culprits behind his sister's murder, and they hunt down members of the Revolutionary Brethren through France, Russia and England.
On the other side of the story, Maximilien Robespierre, Francis Dashwood and the Comte St. Germain work against the King and le Secret du Roi to uncover the mysteries of the Royal Psalms and use the Psalms for their own goals. Lia, as one who was actually able to read the Royal Psalms (and knows their secrets), becomes a pawn for both sides.
If it sounds confusing, that's because it absolutely is. The story isn't entirely fabricated. Le Secret du Roi was a real organization, Francis Dashwood's Hellfire Club did exist, D'Eon was sent to Russia to aid the pro-French faction there, and he also collected information for a potential invasion plan against England. There is plenty of build up to the French Revolution specifically and also the American War of Independence. The storyline surrounding the Royal Psalms and the cult that is formed around them is created for the anime. It gets a little out of hand from time to time and becomes somewhat hard to follow.
The anime is based on a novel by Tow Ubukata, who also had a hand in scripting the manga and in writing the screenplay. The 24 episodes span 6 discs from ADV, though the series now belongs to FUNimation. Each DVD comes with an incredible amount of historical data relating to the characters and episodes included. That's really handy to have, but I suggest ignoring it for the most part as it has little to do with what's going on within the story of the anime. It's good for a history lesson though, just be aware of a few typos scattered here and there (including mis-typed dates, which is a nuisance). There are also interviews, clean credits, and a rather bizarre but professional photo shoot of the title character's voice actors (as D'Eon and Lia) cross dressing as their characters (meaning David Matranga wears a dress and Taylor Hannah a suit or uniform). The English voice acting is excellent as well, particularly for the main cast. It's worth a watch or two (a second go through would probably help in understanding what's going on a bit better); it's certainly got the high production value one would expect from Production I.G. Just don't get hung up on the history and enjoy the story you're given.
Image: D'Eon possessed by Lia.