The plot of this novel by a famous Italian writer, philologist and literary historian dates back to the early seventies of the 20th century, a time when youth riots were still raging in Italy. However, in his own words, philology becomes the “political choice” of the narrator, a student at the Casobona University of Milan: “I came to this as a person who boldly picks up the texts of speeches about the truth, preparing to rule them.” He made friends with the scientific editor of the publishing house "Garamon" Belbo and his colleague Diotallevi, who does not interfere with the age difference; they are united by interest in the mysteries of the human mind and in the Middle Ages. Cazobon writes a dissertation on the Templars; before the eyes of the reader is the story of this knightly fraternity, its occurrence, participation in the Crusades, the circumstances of the trial, which ended with the execution of the leaders of the order and its dissolution.

Further, the novel enters the field of hypotheses - Kazobon and his friends are trying to trace the posthumous fate of the Order of the Knights of the Temple. The starting point for their efforts is the appearance in the publishing house of a retired colonel, confident that he discovered the encrypted Plan of the Knights of the Order, a plan for a secret conspiracy, a rematch designed for centuries. A day later, the colonel disappears without a trace; he is supposed to be killed; this incident itself or the unpleasant residue left over from it separates Casobon from friends. Separation is delayed for several years: after graduating from university and having defended his diploma, he leaves for Brazil as a teacher of the Italian language.

The immediate reason for leaving is his love for the local native of Amparo, the beautiful half-breed, imbued with the ideas of Marx and the pathos of a rational explanation of the world. However, the very magical atmosphere of the country and unusual encounters, which fate brings to him with inexplicable persistence, make Kazobon, for the time being, almost imperceptibly for himself to do the reverse evolution: the advantages of rational interpretations seem to him less and less obvious. He again tries to study the history of ancient cults and hermetic teachings, introducing Amparo to his studies and the skepticism; he is attracted to the land of the sorcerers - Bahia, to the same extent as the lecture on the Rosicrucians, delivered by a compatriot-Italian, by all indications - one of those charlatans, the number of which he has yet to guess. His efforts to penetrate the mysterious nature bear fruit, but for him they turn out to be bitter: during a magical rite in which they were invited to participate in a special arrangement, Amparo, against his own will, falls into a trance and, having regained consciousness, cannot forgive this myself, nor him. After spending another year in Brazil, Casobon returns.

In Milan, he meets Belbo again and through him receives an invitation to collaborate in the publishing house Garamon. At first it was a question of compiling a scientific encyclopedia of metals, but soon his area of ​​interest expanded significantly, again capturing the sphere of the mysterious and esoteric; he admits to himself that it is becoming increasingly difficult for him to separate the world of magic from the world of science: people who were told to him back in school that they carried the light of mathematics and physics into the jungle of superstitions, as it turns out, made their discoveries, “relying on on the one hand, to the laboratory, and on the other, to Kabbalah. ” The so-called Hermes project, the brainchild of Mr. Garamon, the head of the publishing house, contributes a lot to this. Kazobon, Belbo, and Diotallevi are connected to its implementation. Its essence is to announce a series of publications on occultism, magic, etc., to attract both serious authors and fanatics, crazy people who are ready to pay money for the publication of their creations; these latter are supposed to be fused to the publishing house "Manuzio", whose relationship with "Garamon" is kept in the strictest confidence; it is intended for the publication of books at the expense of the authors, which in practice boils down to the merciless “issuing” of their wallets. Among the occultists, Garamon relies on a rich catch and therefore urges Belbo and his friends not to neglect anyone.

However, publications intended for Haramon must still meet certain requirements; as a scientific consultant to the project, on the recommendation of Casobon, a certain gentleman familiar to him from Brazil is invited, either an adventurer, or a descendant of a noble family, perhaps a count, but in any case a rich man, with a delicate taste and undoubtedly deep knowledge in the field of magic and occult sciences; he speaks of the most ancient magical rituals as if he himself were present at them; in fact, sometimes he directly hints at it. Moreover, he is not a snob at all, he does not shy away from obvious charlatans and psychos and is sure that even in the most useless text one can find “a spark of truth, if not an unusual deception, and often these extremes touch”. Hoping to divert the stream with his help, he choked, directing him to enrich his master, and, perhaps, to find in him several grains of truth for himself, the heroes suppressed by the authority of "Mr. Earl" are forced to flounder in this stream, not daring to reject anything: in any tare, there may be a grain that is invisible and not detectable either by logic, or by intuition, or by common sense, or by experience. Here are the words of the poor alchemist, overheard by Kazobon during another ritual that was nowhere near their native homes, this time where they fall at the invitation of Alla: “I tried everything: blood, hair, the soul of Saturn, marcassites, garlic, Martian saffron, shavings and iron slags, lead burl, antimony - all in vain. I worked to extract oil and water from silver; I burned silver with and without specially prepared salt, as well as vodka, and extracted caustic oils from it, that’s all. I used milk, wine, rennet, sperm of stars that fell on the ground, celandine, placenta; I mixed mercury with metals, turning them into crystals; I sent my searches even to ashes ... Finally ...

- What - finally?

- Nothing in the world requires more caution than the truth. To find her is like bleeding right from the heart ... "

Truth is capable of overturning or destroying the world, for it has no protection from it. But truth has not yet been discovered; that’s why one should not neglect anything - it’s better to try again everything that has ever been the subject of the efforts and hopes of any of the initiates. Let unjustified; even if it’s wrong (and what then were they dedicated to?) - it doesn’t matter. “Every mistake can turn out to be a fake bearer of truth,” says Allier. “True esotericism is not afraid of contradictions.”

And this maelstrom of erroneous truths and errors fraught with truth again pushes friends to search for the Plan of the Order of the Templars; the mysterious document left by the disappeared colonel is studied by them again and again, and historical interpretations are sought for each item: it was allegedly carried out by the Rosicrucians, these were the Pavlikians, Jesuits, Bacon, the assassins had a hand here ... If the Plan really exists, it must explain all; under this motto the history of the world is rewritten, and gradually the thought “we have found the Plan by which the world moves” is replaced by the thought “the world moves according to our Plan”.

Summer is passing; Diotallevi is returning from vacation already seriously ill, Belbo is even more enthusiastic about the Plan, good work on which compensates for his defeats in real life, and Cazobon is preparing to become a father: his new girlfriend Leah should soon give birth. Their efforts, meanwhile, are nearing completion: they understand that the place for the last meeting of the participants in the Plan should be the Paris Museum in the church of the abbey of Saint-Martin-de-Chan, the Repository of Arts and Crafts, where the Foucault Pendulum is located, which at a strictly defined moment will point them a point on the map is the entrance to the possessions of the King of the World, the center of telluric currents, the Navel of the Earth, Umbilicus Mundi. They gradually assure themselves that they know the day and hour, it remains to find a map, but here Diotallevi finds himself in the hospital with the most disappointing diagnosis, Cazobon leaves with the baby and Leah in the mountains, and Belbo, driven by jealousy of Alia, who made him a happy rival in his personal life, he decides to share with him their knowledge of the Plan, keeping silent about the absence of a map, and the belief that all this decoding is not the fruit of their general raging imagination.

Leah, meanwhile, proves to Casobon that the fragmentary records of the end of the 19th century that they took as an outline of the Plan are most likely the calculations of the owner of the flower shop, Diotallevy at death; his cells refuse to obey him and build his body according to his own plan, whose name is cancer; Belbo is in Alie’s hands and a pack of like-minded people, first finding a way to blackmail him, then luring him to Paris and forcing them, under pain of death, to share with them the last secret - a map. Cazobon rushes to search for him, but only manages to find the final: in the Vault of Arts and Crafts, there is a frantic crowd of alchemists, Hermeticists, Satanists and other Gnostics led by Alieu, here already, however, called Count Saint Germain, desperate to obtain recognition of the location of the map from Belbo , executes him, crushing him with a rope tied to the Foucault Pendulum; while his beloved dies. Cazobon flees; the next day in the museum there are no signs of yesterday’s incident, but Cazobon has no doubt that now it will be his turn, especially since when he leaves Paris he learns about the death of Diotallevi. One was killed by people who believed in their Plan, the other - by cells who believed in the ability to compose their own and act on it; Cazobon, not wanting to endanger his beloved and the child, locks himself in the Belbo house, flips through other people's papers and waits for who and how will come to kill him.