d'Eon de Beaumont is a police officer who is a member of King Louis XV’s Secrete Police.
At one point he becomes involved in looking into the murders of virgins that are taking place around Paris, the murders it is revealed are being committed by a cult called The Poets who use the blood of the virgins to write poems that let them turn into monsters.
During the investigation Beaumont’s body is from time to time taken over by the spirit of his dead sister Lia de Beaumont, during these periods, he dresses as his sister and his body transforms into his sister's and as his agility, and swordsmanship improves greatly which helps as he clashes with The Poets as Le Chevalier Sphinx.
There is also a 24 episode anime series based loosely on the manga that used the same characters, but changed the story and theme greatly, while the manga was played mainly for humor and there seems to be no reason for his sister to possesses his body, the anime is done as a historic fiction swashbuckler with fantasy and horror elements, among other changes.
In the anime d'Eon de Beaumont’s sister Lia de Beaumont, who is a spy for the king, is found in a coffin with the word “Psalms” written on it floating down the Seine, when taken out of the river it is found that her body has been filled with mercury.
d'Eon de Beaumont, a member of the King’s Secret Police, is part of the investigation.
At one point during inquiry the rest of the members of the Secret Police that Beaumont is with are killed by a mercury filled vampire like creature that almost kills him as well until it retreats on touching a cross that he had taken off his sisters body.
It is during the battle with the monster that the vengeful spirit of Lia de Beaumont takes over his body transforming him into an imager of her while doing so.
After that d'Eon de Beaumont carries on the search for the truth behind his sister’s death, the monsters that killed his fellow policemen, and the alchemists and revolutionaries behind it all with the help of new companions, and the spirit of his sister.
The Manga and Anime are based VERY loosely on the life of Charles Geneviève Louis Auguste André Timothée d'Éon de Beaumont (5 October 1728 - 21 May 1810), usually known as the Chevalier d'Éon, the famous French diplomat, spy, soldier and Freemason who lived the first half of his life as a man and the second half as a woman, not because he wanted to, but because of some complicated politics and royal edits of the time.
This version of Le Chevalier d'Eon was depicted in the pages of Spy-Hunters from American Comics Group.