The plot of this novel comes at the beginning of the seventies the twentieth century, when in Italy youth riots were still raging. However, philology became the "political choice" for the narrator, a student at the University of Milan Casaubon. He became friends with the scientific editor of the publishing house "Garamon" Belbo and his colleague Diotallevi, who does not mind the age difference. They share an interest in the mysteries of the human mind and in the Middle Ages. Casaubon wrote a thesis on the Templars; before the eyes of the reader passes the history of this chivalrous brotherhood, its origin, 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 comes to hypotheses - Casaubon with friends is trying to trace the posthumous fate of the Order of Knights of the Temple. The starting point for their efforts is the appearance in the publishing house of a retired colonel, who is confident that he had found the encrypted plan of the Knights, conspiracy plan, plan of revenge, designed for ages. A day later, the colonel disappeared without a trace; it is assumed that he was killed; the incident itself or its unpleasant aftertaste makes Casaubon separate with his friends. Separation has lasted for several years: after finishing university, he goes to Brazil as a teacher of Italian language.
The immediate reason for leaving is his love of the local native Amparo, the beautiful half-breed, imbued with the ideas of Marx and pathos of the rational explanation of the world. He tries again to study the history of ancient cults and hermetic teachings, attaching to his studies skeptical Amparo; he is attracted to the land of witches - Bahia. His efforts to penetrate into the mysterious bring its fruit, but they are bitter for him: during magical ritual, to participate in which they with Amparo were invited, against her will she falls into a trance and, waking up, can forgive this neither herself nor him. After another year in Brazil Casaubon returns.
In Milan, he again meets Belbo and receives an invitation to work in the publishing house "Garamon". First, it is a question of drawing up a scientific encyclopedia of metals, but soon the sphere of his interests significantly expands again capturing the mysterious and esoteric; he admits to himself that it becomes more difficult to separate the world of magic from the world of science. There comes a series of publications on occult and magic, to attract serious sponsors and fanatics willing to pay the money for the publication of their works.
However, the publication intended for "Garamon" must still meet certain requirements; as a scientific consultant of the project, on the recommendation of Casaubon Mr. Allier is invited. He is from Brazil, rather rich, with a delicate taste and undoubtedly with a deep knowledge of magic and the occult; he tells about the most ancient magical rituals, as if he has been present himself; in fact, sometimes he hints at this.
These events push friends again in search of the Plan of the Templars; mysterious document, the remainder of the missing colonel, is examined them again and again, and its each point seeks for historical interpretation. If the plan does exist, it must explain everything.
The summer passes. Diotallevi returns from vacation seriously ill, Belbo is even more thrilled by the Plan, and Casaubon is preparing to become a father: his new girlfriend Leah is soon to give a birth. Their efforts, meanwhile, are approaching the end, they realize that the last meeting of the Plan participants should become a Parisian museum in the abbey church of Saint-Martin-de-Shan, Repository of Arts and Crafts, where Foucault's Pendulum, which at a certain moment would show them the point on the map - the entrance to the possessions of King of the World, the center of the telluric currents, Navel of the Earth, Umbilicus Mundi. They gradually assure themselves that they know the day and hour, it is only find a card, but then Diotallevi is in the hospital with the most disappointing diagnosis, Casaubon goes along with Leah and the baby to the mountains, and Belbo, driven by jealousy to the Allier, who became his happy rival in his personal life, decides to share with him their knowledge of the Plan, but remaining silent about the absence of maps, and confidence in the fact that all of this transcript is not the fruit of their total raging imagination.
Leah meanwhile proves Casaubon that those fragmentary records of the end of the XIX century, they took over the outline of the Plan, are likely to be a flower shop’s owner’s accounts. Diotallevi is at death, it iscancer; Belbo is in the hands of the Allier and a pack of his followers, who blackmail him, and under the pain of death force him to share with them the last secret - map. Casaubon rushes to find him, but manages to catch only the finale: the Repository of Arts and Crafts frenzied crowd of alchemists, Hermetic, Satanists and other Gnostics led by Allier, who is called now the count of Saint-Germain. Desperate to get the Belbo’s confession they hang him to the Foucault pendulum; his beloved also dies. Casaubon flees; the next day at the museum there is no trace of yesterday's incident, but Casaubon has no doubt that now they will be after him, besides Diotallevi also dies. Casaubon, not wanting to jeopardize his beloved and child, locked himself in the Belbo’s house, waiting for someone, who will come to kill him.