"Le Chevalier d'Eon" is a historical fantasy manga series set in 18th-century France, blending real historical events with supernatural elements. The story begins with the mysterious death of Lia de Beaumont, a young woman whose body is discovered in a coffin floating in the Seine River. Her brother, d'Eon de Beaumont, a skilled knight and diplomat, becomes determined to uncover the truth behind her death. Lia’s spirit, however, does not rest; it manifests within d'Eon, allowing her to possess his body and aid him in his quest. This duality becomes central to the narrative, as d'Eon and Lia share a single form, with Lia often taking control during moments of danger or emotional intensity.

The investigation leads d'Eon to uncover a dark conspiracy involving the royal court, secret societies, and supernatural forces. The Psalms, a mysterious and powerful book, becomes a focal point of the plot. It is said to grant immense power to those who possess it, and various factions, including the French monarchy, the Russian Empire, and a shadowy organization known as the Revolutionary Order, vie for control of it. The Psalms are linked to a series of occult rituals and the reanimation of the dead, creating an atmosphere of intrigue and danger.

As d'Eon delves deeper, he encounters a diverse cast of characters, both historical and fictional. Among them are King Louis XV, who plays a significant role in the political machinations of the era, and the enigmatic Comte de Saint-Germain, a figure shrouded in mystery who aids d'Eon in his mission. Robin, a young orphan with a tragic past, becomes a loyal companion to d'Eon, providing both emotional support and practical assistance. The narrative also explores the complex relationships between d'Eon, Lia, and their shared existence, as well as the broader themes of loyalty, identity, and the cost of power.

The story unfolds across multiple settings, from the opulent halls of Versailles to the war-torn streets of Paris and the icy landscapes of Russia. The political turmoil of the time, including tensions between France and England, serves as a backdrop to the personal and supernatural struggles of the characters. The manga incorporates elements of espionage, sword fights, and occult rituals, creating a rich tapestry of action and intrigue.

Throughout the series, d'Eon’s journey is marked by a series of revelations about the true nature of the Psalms, the extent of the conspiracy, and the sacrifices required to protect those he loves. The interplay between historical events and fantastical elements provides a unique narrative framework, blending fact and fiction to create a compelling and immersive story. The manga’s exploration of themes such as justice, revenge, and the blurred lines between life and death adds depth to its portrayal of 18th-century Europe, making "Le Chevalier d'Eon" a distinctive entry in the historical fantasy genre.