In the year 1349 of the Fifth Epoch, in a world reminiscent of the Victorian era, a Chinese man named Zhou Mingrui is transported into the body of Klein Moretti, a university student who has just committed suicide after being bewitched by an evil god. Now inhabiting a reality where steam-powered machinery, cannons, dreadnoughts, and airships coexist with potions, divination, curses, and sealed artifacts, Klein must adapt to survive. His initial goal is simple: find a way back home. To investigate the circumstances of his transmigration, he replicates the mysterious ritual that seemingly caused it. Instead of returning to Earth, this act transports his consciousness to a mysterious space above the gray fog, where he gains strange abilities and inadvertently impersonates a powerful, ancient entity. Adopting the title The Fool, he establishes the Tarot Club, a weekly gathering where he invites individuals from across the world, hiding his own weakness behind a mask of omniscience and omnipotence.

The world Klein enters is a intricate blend of steampunk aesthetics, Lovecraftian horror, and Chinese xuanhuan fantasy. It is governed by a complex power system centered on Beyonders, individuals who gain supernatural abilities by consuming magical potions. These potions are organized into twenty-two distinct divine pathways, each named after a tarot card and composed of ten sequences, counting down from Sequence 9 at the lowest to Sequence 1, with Sequence 0 representing a god. To advance, a Beyonder must not only consume the next potion but also digest it by acting in accordance with the name of their sequence, a secret technique known as the Acting Method. Failure to properly digest a potion or advancing too quickly leads to a loss of control, transforming the individual into a dangerous, inhuman monster. Official organizations like the Nighthawks, to which Klein becomes affiliated, exist to manage these supernatural threats and regulate Beyonders.

As Klein delves deeper into the world's secrets, his journey unfolds across multiple distinct narrative arcs. In his first role as a member of the Nighthawks in the city of Tingen, he uncovers a conspiracy that leads to the tragic death of his team captain and his own apparent demise. Faking his death, he leaves Tingen and travels to the great metropolis of Backlund, where he assumes the identity of the private detective Sherlock Moriarty. Here, he works to stop the devastating Great Smog event, a ritual orchestrated by secret forces to cause mass death. Forced to flee Backlund, Klein adopts the persona of the ruthless and legendary adventurer Gehrman Sparrow, hunting pirates on the high seas to gather rare ingredients for his advancement to a demigod. He later returns to high society as the wealthy philanthropist Dwayne Dantès, where he successfully prevents King George III from ascending to godhood and exposes a plot for the resurrection of the Black Emperor.

Throughout his journey, Klein is guided by the knowledge that he is not just a Beyonder but also a vessel for a cosmic entity known as the Lord of the Mysteries. His final challenge comes in the forsaken land of the gods, where he engages in a high-stakes game of deception and pursuit with Amon, a mischievous and terrifying Angel of Time and the son of the ancient sun god. In a desperate struggle for control over the Sefirah Castle, the source of his gray fog powers, Klein ultimately succeeds in his advancement. He becomes the new Lord of the Mysteries, a Sequence 0 deity, taking control of the pathways of The Fool, The Error, and The Door, and ascending to the position of a Pillar of the universe to protect the world from the influence of even greater, outer cosmic horrors.