Adso, a young German soldier who wishes to take his vows, meets William from Baskerville, a cultured and intelligent Franciscan monk, and decides to follow him. The two head to a Benedictine Abbey in the Alps, famous for its library, where a dispute will take place on the role of the Church and its wealth. At their destination, William and Adso discover that the abbey is in full commotion following the death of Adelmo, the miniaturist of the scriptorium: is it murder or suicide?

On the order of Abbot Abbone, and while the death continues to upset the life of the abbey, William asks the monks to try to shed light on the facts and all traces lead straight to the library, a sort of inaccessible maze of which only the librarian knows the secrets. Meanwhile, the brutal inquisitor Bernardo Gui, directed to the abbey with his armed escort, performs a bloody massacre in a refuge of Dolciniani heretics, exterminating the family of Anna, daughter of Dolcino and Margherita.

Adso was the victim of a drug: one of the many tricks used to stop unwanted visits to the library. And, while William continues his investigations, the novice cannot resist the charm of the red-haired girl and flees again to meet her. Inquisitor Bernardo Gui is increasingly obsessed with the memory of Margherita and his trial, just as Anna tries to kill him.

The autopsy on Berengario's body reveals that the monk died drowned, but William discovers strange black spots on his fingers and tongue, similar to those seen on Venantius's corpse. During the night, Baskerville and Adso succeed in entering the library again, in search of the "Finis Africae" mentioned in the code of the dead miniaturist: is it perhaps the room where the key to the crimes is hidden?

In the forest, the Occitan girl finds Anna wounded and unconscious and caring, restoring her strength. Meanwhile, at the abbey, the climate between the two delegations is increasingly hostile and wary. Informed of what is happening, Bernardo Gui begins to investigate Remigio, sure he has already met him in the past. Thus begins the dispute over the riches of the Church, with the Franciscans having a clear suspicion that the Pope is plotting to declare their doctrine heretical.

Anna, thanks to the fog, dresses up as a boy and manages to infiltrate the abbey. The woman is looking for Remigio to ask him to give her back the letters that Dolcino had given him before being arrested and killed. But the letters have disappeared and probably ended up in the wrong hands. Severino, the herbalist monk, tells Guglielmo that he found a strange book in his laboratory: is it the memories of Dolcino or something else?

The arrest and the trial of Remigio help Bernardo Gui to make the Dispute fail and to involve the Franciscans themselves in the charge of heresy. The cellarer is subjected by the inquisitor to a harsh interrogation that forces the monk to confirm all the accusations that are made against him. Malachi, meanwhile, managed to get his hands on the missing book and bring it back safely to the library.

The death of Malachi shows everyone that the investigations carried out by Bernardo Gui have not served to unravel the mysteries that surround the abbey. Adso does not give himself peace for the girl's fate and asks William to do everything to help her. Baskerville decides to talk to the inquisitor, who initially seems to be listening to him but has actually already made a decision. After a hard confrontation with the abbot, the English monk has no choice but to solve the enigma of the "Finis Africae".