Jacopo Belbo, the senior editor at a publishing house in Milan, Italy, is taken hostage by a shadowy group of occultists. These Diabolicals, as Belbo thinks of them, are convinced that he possesses a secret map or code. They are determined to get hold of it so they can complete the task they believe their society was set up to accomplish. Casaubon, the firm’s junior editor, rushes to Paris, hoping to save Belbo’s life and perhaps his own. He hides in the Musée des Arts et Métiers (Museum of Arts and Trades), where the famous pendulum of Léon Foucault is housed. Casaubon’s investigations have led him to believe that the occult society will gather there at midnight that night, as the moment of the summer solstice approaches.

As he waits in hiding, Casaubon thinks over the events of the last dozen years: He first meets Belbo when in a Milan tavern that serves as a meeting place for students and workers of every political persuasion; it has always seemed to him to resemble Rick’s bar from the 1942 film Casablanca. He is then a doctoral student in philology, writing a thesis on documents pertaining to the medieval Knights Templar. Belbo is reviewing a manuscript submitted by a retired army colonel that purports to solve the mystery of the Templars’ lost treasure. Just as Casaubon is brought into the office, where he briefs Belbo and the other editor, Diotallevi, on the Templars, the manuscript’s author disappears, and the three men of letters become caught up in a police investigation.

Casaubon leaves Italy shortly afterward, following a beauty named Amparo back to her native Brazil, where he teaches for the next two years. Returning to Florence in the late 1970’s, he is unable to secure a teaching position and therefore becomes a literary researcher, modeling his agency on that of Sam Spade, the detective hero of the 1941 film The Maltese Falcon. He is soon doing research for Belbo’s firm and before long is a regular fixture in the editorial office. He marries and awaits the arrival of his first child. Then disaster strikes.

An adviser to the press, an old occultist who hints that he has been active for several hundred years, invites the editors to a Rosicrucian ritual in a country villa. They attend and witness the mysteries not forbidden to outsiders, but Belbo is shocked to learn that the old man is after his beautiful mistress and seems to have won her affection. Madly jealous, Belbo later decides to make the old man envy him. He therefore intimates that he has the secret map or code that seemed to have been lost when the colonel disappeared some years earlier.

Belbo soon regrets his rash claim. He had thought the search for this code was an intellectual game. The three editors devised rules on their own for deciphering the code and figuring out what they called “the plan.” They never suspected that people would take the game so seriously that they were prepared to kill.

By the time Belbo disappears, Diotallevi is in the hospital. He is dying of cancer, half-convinced that the deadly cells multiplying in his body represent a sort of divine judgment on his efforts to crack the Kabbalistic code. Meanwhile, Casaubon has decided that the new life growing in his wife’s womb is far more magical and important than any occult plan. When he goes in search of Belbo, he is trying to protect both their safety and their sanity.