Casaubon, an Italian history graduate, gets a job with a publisher of occult books. There he helps his two colleagues, Jacopo Belbo and Diotallevi, in assessing submitted manuscripts about the Knights Templar, an order of monk crusaders that were supressed in 1344, but about which conspiracy theories have grown that say they survived as a secret society and control history. Casaubon and Belbo meet an array of 'occultist' would-be authors, each with their own mad theory. Madness is piled upon madness; conspiracy theory upon conspiracy theory. Then, for their amusement, Casaubon and Belbo start to invent the conspiracy theory to end all conspiracy theories: it links the Templars, Francis Bacon, the Rosicrucians, the Freemasons, Napoleon Bonaparte, Tsar Nicholas, Adolph Hitler, and has at its heart is a great secret concerning 'underground currents'. But Casaubon and Belbo are sucked into the madness when the occultists start to believe in their theory and want to know the secret.
This a really biting satire of occultists, occultist thinking and occult publishing. It is also rather cleverly written and very funny in parts.