Casaubon: Protagonist and narrator. Also an intellectual dilettante and expert on The Knights Templar.
Jacopo Belbo: Editor who is haunted by failure and frustrated desires.
Diotallevi: Belbo's partner who is obsessed with all things Jewish and Kabbalistic.
The narrative contains numerous flashbacks, dream sequences, and historical anecdotes, but the basic plot is this:Casaubon, while working on his degree, meets Belbo at a bar one night and they have a conversation during which Casaubon reveals his field of expertise - the history of the Poor Fellow-Soldiers of Christ and of the Temple of Solomon, also known as the Knights Templar. Belbo, who works for a publisher named Garamond who specializes in academic works and doubles as a vanity press for various crackpots and assorted nuts with pretensions to intellectualism, finds Casaubon interesting and invites him to visit the publisher. Casaubon is introduced to Diotallevi and is allowed to be present and re-presented as a "topical expert" while they interview an aspiring author, Colonel Ardenti, who presents a manuscript. This turns out to be a book purporting to expose the secret of the Templars, which Ardenti believes to be dangerous knowledge.Sure enough, the man disappears mysteriously, but the police investigation goes nowhere. Casaubon moves to Brazil, where he hooks up with a radical socialist girlfriend named Amparo and later meets a man named Aglič who is an expert in occultism. Aglič likes to tell historical stories as though he were there in person. Eventually, Amparo leaves Casaubon, and Casaubon moves back to Italy.Upon his return, Casaubon becomes a freelance researcher and accepts work from Garamond, first on a history of metals, and then a history of occultism. He also meets and falls in love with a woman named Lia. During their projects, he, Belbo, and Diotallevi meet many odd characters from the occult world of conspiracy theories and secret histories, partly with the help of Aglič who has contacts there. Bemused and bored by the inanity of the various conspiracy theories they are require to listen to, read about, and publish, the trio decide one night to fabricate their own arcane history based on the idea that one root occult conspiracy is the cause of all world historical events. This byzantine, chimerical super conspiracy they enigmatically designate "The Plan".Over Lia's objections, Casaubon and his partners become more and more invested in the Plan they have created, but then unwisely start hinting to Aglič that they possess knowledge he does not. Aglič, Ardenti, and other members of the European occult community decide that they are meant to be in control of history, and start chasing after the (completely synthetic) secret of the Plan, starting by pursuing the trio of protagonists themselves.
Arson Murder And Jay Walking: A really peculiar inversion with the garrulous publisher's habit of starting out hyperbolic in his praise of things and then (apparently without realizing it) taking an abrupt downturn. E.g. "It is a palace! A dwelling fit for kings! I'll put it even more strongly: It's a genuine Piemontese villa!"
An example done straight is in when Casaubon mentioned the Borborites, who rip out fetuses from women's bodies, crush them in mortars and eat them with honey and pepper. Diotellavi says: "How revolting, honey and pepper!"
Loving Force: Subverted. When Amparo, as she told, noticed a guy approaching her in the night on the street, and suspected he wanted to rape her, she offered him to have sex. He ran away, because he wanted to hurt women, not sex.
No Name Given: The first names of Casaubon and Diotallevi are never revealed.
Casaubon's first name may be Pim. However, only Lia calls him that, so it might be just a nickname.
Numerological Motif: Lia gives Casaubon a talk about the real meanings. The number one is special because every human is, well, one human and has one head, heart, nose, mouth, private part etc.; two is special because two people make a couple, and humans generally have two hands, feet, eyes, ears etc.; three OTOH is so special because our bodies don't have three of anything (but man + woman + child make a family); etc.
Written in a narrative style with Belbo imagining himself as a reincarnation of the true original writer.
Shout-Out: Hundreds if not thousands of them. Belbo's files are especially crammed: one reads like a crazy Troperiffic pastiche in which each paragraph (maybe each line) references a different nineteenth century adventure, mystery or conspiracy story. Did we say Genius Bonus?
Stylistic Suck: Belbo's own attempts at litterature are quite awful. He is very aware of this and doesn't want anyone to read them. His only "good" pages are when he abandons the Plan for his chilhood memories.
Take That: A strong one against conspiracy theories and those who believe in them.
Taxidermy Is Creepy: The narrator meets the taxidermist Salon in his shop and feels very creeped out.
The Password Is Always Swordfish: Played with. Casaubon tries to figure out the password to Belbo's computer, which asks: "Do you know the password?" Since Belbo is his close friend, he tries numerous expressions he thinks Belbo could've used, but none of them work. Eventually, he angrily types: "No." This is the password. (There's a deeper reason for this: In order to gain knowledge, you have to admit that you don't know a specific thing.)
Vanity Publishing: In-story, there is the publishing house of Garamond doing this. The owner also changes the business model: he makes the occultists and conspiracy nutters pay to have their books published, and then sells the books.