Unusual for an anime -- or, really, pretty much any TV show -- this is set during the French Revolution. It's an interesting combination of fantasy and historical fact.

Initial Impression:

The packaging on this is classy -- there's a matte black slipcover with gold leaf, plus a very nice insert inside with interviews, bios and pics of the characters, and various other goodies. D'Eon de Beaumont, on the cover, also bears a striking resemblance in the pose (sword in front of face, eyes slitted) to Rurouni Kenshin -- and it turns out this is no coincidence because Le Chevalier's director, Kazahiro Furuhashi, also directed RuroKen. He doesn't disappoint with this new project.

(D'Eon has no similarity to Himura Kenshin beyond the pose, just to be clear. But that stance on the cover, plus a few moments in the show itself, bear a striking resemblance to the Hitokiri Battousai. This is apparently deliberate based on a confirmation in the commentaries on the DVD. It's a nice detail for observant fans and doesn't detract at all from the show.)

The series itself has a very sharp look. The animation feels like 1700's France. The characters' costumes are gorgeously drawn with rich detail and are reasonably authentic to the time. I suspect the real clothing would have been a quite a bit more elaborate, to the point of being ridiculous, because the French court of this time was not known for good taste ... However, the character costumes are gorgeously detailed, work well, and I have no complaints with them.

But, a few things do bug me -- the faces don't feel right sometimes, particularly the mouth anatomy (the dubbing is fine) and the transitions between CGI backgrounds and animation and 'cel' style animation is jarring.
Overall, however, it's beautifully done. And the 'mouth anatomy' observation comes from the fact that the the animation style is very realistic.

The voice actors are very good in the English dub. (As per usual, I haven't watched the Japanese version. This is because reading subtitles gives me a vicious headache.) David Matranga as D'Eon de Beaumont is awesome, and the fifteen year old Tyler Galindo does notably well as the page Robin. Other characters are equally well acted.

So, the first impression is very favorable. On to characters and plot ...

Many of the characters in this show, including D'Eon, are based on real historic people -- with varying degrees of authenticity. In D'Eon's case, he is based on a nobleman who was known to be a spy and who lived much of his life as woman.

The anime takes historical fact and runs with it. The D'Eon in the series is possessed by the spirit of his (fictitious) sister, Lia, who is well voiced by Taylor Hannah. Lia was murdered and the blood in her body replaced by mercury (a process that the bad guys use to create "gargoyles"); because this preserved her body and prevented it from decaying, her spirit is unable to rest. Angry and vengeful, her spirit takes over D'Eon's body when he is threatened. She is, apparently, a better swordfighter than he.

(The real D'Eon would win bets by dueling in the guise of a woman. He also purportedly used the name 'Lia' when he went undercover as a woman in the Russian court.)

The transformation between D'Eon and Lia is nicely handled -- while the character design doesn't change much, D'Eon's facial features become just a little bit softer, and his very long blond hair falls into a 'feminine' style.  The character moves differently, as well, walking like a woman rather than a man.