"Rex Mundi" is a manga series set in an alternate history where the Catholic Church retains significant political and religious power in Europe during the 1930s. The story unfolds in a world where magic is real, and the Church wields it as a tool of control. The narrative centers on Dr. Julien Saunière, a physician and occult investigator in Paris, who becomes embroiled in a complex conspiracy after the murder of his friend, Father Gérard. The investigation leads Saunière to uncover a hidden history involving the Holy Grail, the Merovingian bloodline, and a secret society known as the Rex Mundi.

As Saunière delves deeper, he discovers that the Grail is not a physical object but a symbol of a powerful bloodline tied to the descendants of Jesus Christ and Mary Magdalene. This revelation threatens the Church's authority, as it challenges the foundation of its doctrine. Saunière's quest for the truth brings him into conflict with the Church, secret societies, and powerful individuals who seek to control or destroy the Grail's legacy. Along the way, he encounters allies and adversaries, including Geneviève, a mysterious woman with her own ties to the Grail, and Moricant, a ruthless Church enforcer.

The plot intertwines historical elements, religious mythology, and occult themes, creating a rich tapestry of intrigue and suspense. The story explores themes of faith, power, and the manipulation of history, as Saunière navigates a world where the line between truth and deception is constantly blurred. The series builds toward a climactic confrontation that reveals the true nature of the Grail and its implications for the future of humanity. Through its intricate plot and well-developed characters, "Rex Mundi" offers a compelling exploration of alternate history and the enduring influence of myth and religion.