The book begins with an unnamed author who talks about a manuscript he has found but a manuscript he can’t say for sure if it is a forgery or a true story. The narrator then presents various proofs that made him believe the manuscript to be true and that made him think that maybe the story was true after all.
The book then presents a prologue told by Adso who writes sometimes at the end of the 14th century. Adso tells the reader that he will write about a series of events that took place in 1327, when he was 18 years old and a fresh monk in the Benedictines order.
Adso presents the historical context and tells the reader how during his time, there were disagreements between the Pope and the Emperor. The situation got so tense that the Pope excommunicated the Emperor. The Franciscans sided with the Emperor and thus Adso’s father decided to take his son from the monastery and send him to Italy. To avoid sending him on his own, Adso is made the servant of William of Baskerville, a Franciscan friar dorm England. Adso likes his new master as he is different from everyone he knew until then and because he is interested in the technological advancements of the age.
Adso then describes the events that take place over the course of seven days. On the first day, Adso and William arrive at an abbey where they were supposed to meet with people representing the Emperor and the Pope. William and Adso meet with Remigio of Varagine, the cellarer, who tells that that a horse has been lost. William finds the horse of Abo of Fossanova, the abbot who then later comes to talk with them.
At the abbey, Abo asks William for help and tells him a strange event that took place in the abbey. Abo tells William that a man felt from the Aedificium but when the tower was inspected, all the windows were found to be close shut. Abo fears that either someone else from the abbey pushed the man or that some evil spirit was the one responsible for the man’s death. William agrees to help but only if he is allowed to move freely in the abbey and question whom he wants. Abo grants him the freedom to move in the abbey but forbids him of entering the library, claiming that it is too dangerous.
A new character is introduced, a man named Salvatore of Monteferrat, a monk who is strange both in appearance and in manner. William shows Adso another man, named Ubertino of Casale, a man who was almost killed by the Pope because of his views about the value of goods and the position the church should take regarding the possession of wealth. Ubertino and William were friends in the past and Ubertino tells William that he no longer associated himself with the Spiritualists because they promotes free love and were for sexual relationships outside the bound of marriage. Ubertino criticizes William for his actions and claims that he should have been more determines as a judge for the Inquisition. Ubertino also tells William that the abbey’s cellarer and Salvatore were also followers of groups considered as being heretic.
William is introduces to the other people living in the abbey and he meets Severinus, the herbalist and the person in charge of the infirmary, Malachi, the librarian and his assistant, Berengar. A translator named Venantius suggests that Berengar and Adelmo, the monk who died were close friends but Berengar denies those claims.
William then talks with Malachi and he is pleased to find that Malachi shares his ideas about technology and about the idea that a person should not be afraid to use technology for his own good.
That night, William argues at dinner with other monks about laughter and about whether a person is allowed to laugh or not. That night, William tries to break into the library to prove that someone could enter it even though they did not had a key.
The next day, the dead body of Venantius, the translator is found in a barrel filled with pig’s blood. William deducts that the translator was killed somewhere else and then his body was dragged to the barrel and the herbalist confesses that he has herbs that could kill a person and that maybe some of those herbs were used to kill the translator.
William then talks with Benno who tells him that he heard an argument between the translator and Jorge a few days earlier in the library about laughter and about a forbidden book named the ‘’finis Africae’’. Then, William and Adso see an argument between Salvatore and Remigio about who is a heretic.
William goes back to talk with Jorge about laughter and the reason why he refuses to believe that Jesus laugh during his life time. Jorge refuses to listen, claiming that Jesus never expressed joy because laughter is just another way through which a person can mask his or her doubt.
That night, Benno tells William that the librarian and Adelmo began sleeping together when Adelmo offered sexual favors in exchange for access to the secret books. Benno then theorizes that Adelmo killed himself because he was feeling too guilty for what he has done.
Abo tells William that it is important for him to solve the mystery because the event also has political implications and not solving the mystery could result in the abbey being taken over by Governmental powers. Abo fears that some men in the abbey, like Remigio who can be considered as having ties with heretics will be accused with the murder and that the group linked with the person in question will be accused as well.
Another monk tells William that he can enter the library through a secret entrance from the crypt but before they enter the library they hear sounds from inside it, hinting thus that someone was there already. While William and Adso were looking around in the library, someone stole a book as well as William’s glasses and while Adso tried to run after the thief, he was unable to catch him. Adso and William continue their journey in the library but soon find that they are unable to find their way as the library was indeed, just like a labyrinth. While in the library, Adso begins having visions before collapsing but William tries to assure him that most likely, someone put herbs capable of inducing hallucinations in the various rooms in the library and that nothing extraordinary was happening. Adso and William spent a few more hours trying to find their way back and when they did, they were meet by Abo, telling them that another person in the abbey disappeared. The person in question was Berengar, the librarian’s assistant.
Upon investigating his disappearance, William and Adso found only a bloody cloth left behind. Adso begins thinking about the reasons why the Church does not want the rest of the world to have access to their books and theorizes that they are afraid they will lose their influence if they were to make knowledge available to other people as well.
Adso talks with Salvatore and finds that the monk fled his village to escape poverty and then wandered all over Europe until he stumbled upon the abbey where he decided to remain. Adso reaches the conclusion that Salvatore is not really a heretic but rather a person who was easy to take advantage of and thus he went with the group that suited him best. William agrees with him and expresses his opinion about how it became harder and harder to distinguish between who is a heretic and who is not.
William then talks about religion and how he believes that one day, science will replace it and change the society in ways religion could never change it. William discovers a clue, telling them that they need to return to the library and to avoid getting lost, William tries to draw a map by observing the shape of the library from the outside. William also comes with a plan to understand the word written above the various doors in the library.
William then talks with Ubertino about Fra Dolcino, a heretic whom was followed both by Salvatore and Remigio. Fra Dolcino was killed because he preached a simple lifestyle and because he was against the church and its opulence. Ubertino also warns William and Adso about the dangers of women and how they can easily seduce any men they want.
Adso decides to return alone in the library to read about various people who were killed because they were considered as being heretics. Adso begins having visions once more so he returns to the kitchen where he meets a young girl from the village. Attracted by her beauty, Adso has sex with her and then falls asleep.
When William finds him, Adso confesses everything he did but instead of criticizing him, William forgives him. William also theorizes that some girls from the villages came to the abbey and offered sex in exchange for food.
William and Adso then decide to go to the bath and there they discover Berengar’s body, in one of the tubs. Severinus confirms that Berengar died of drowning but that he was most likely affected by the same poison that killed Venantius. Salvatore also confesses that sometimes, girls from the village come to the abbey and have sex with Remigio in exchange for food and that he was the one who found Venantius’s body and moved it from the kitchen since he believed that the body will incriminate him. William also finds his stolen glasses in Berengar’s pocket and then he returns to his room.
That same day, the Pope’s convoy reaches the abbey and William participates at a conference between three men, Jerome of Kaffa, Ubertino and Michael but the meeting is not productive at all and William doubts that he will solve something with the men coming to the meeting.
William also has problems with getting along with the monks at the abbey who criticize him for not being harsh during the time he was a judge for the Inquisition. William and Adso return to the library where they realize that ‘’finis Africae’’ is a room that unfortunately is hidden. When they return from the library, they see Salvatore and the girl Adso slept with being interrogated. It was determined that the girl was a witch and thus she was sentenced to death.
William is told by Severinus that Bernard was in the infirmary the night before he died. A few monks overheard William and Severinus talk and Adso was ordered to follow them.
William then talks with the convoy and proposes that the Church withdraw completely. William also claims that the Church should have no earthly goods since Jesus and the apostles did not have anything either and Bernard urges William to tell the Pope in person his ideas.
After the discussion, William and the others discover the dead body of Severinus, the doctor, inside the infirmary. Remigio was also found in the infirmary and he was arrested almost immediately. William and Adso start looking for the strange book Severinus mentioned before he was killed but they realize that the book was stolen once more.
Remigio is put on trial and under pressure, he confesses both to being a heretic and of committing the murders that took place in the abbey. Remigio is sentenced to death as well as Salvatore who is accused of Remigio of being his accomplice. That night, Michel, an ally of the Pope flees from the abbey helped by Ubertino and Benno admits that he was the one who stole the book and who returned it to the library. That night, William hears a sermon told by Jorge about how wanting to know more is a sin and should be avoided. After the sermon was over, Adso went to bed.
The next day, during the morning prayers, Malachi falls to the ground and dies. William notices that just like the others before him, Malachi had black fingers and concludes that the men who were killed must have all died in the same way. William also concludes that the next victim will also know Greek since every person who died until then knew Greek. Nicholas is chosen as the new cellarer and he talks with William, telling him that the abbey is extremely wealthy and that the men who were killed were all strong candidates for the role of abbot. Adso has yet another vision and based on that vision, William figures out where the finis Africae is located.
William checks the catalogues in the library and finds a book named Coena mentioned in it. When William asks Benno about the book and he confesses that he saw the book and that it was written in four different languages. After the conversation, William reaches the conclusion that someone in the abbey must be killing everyone who knows anything about the book in order to protect its secret. When William talks with the abbot about his theory, he gets offended as William accused him indirectly of being the killer. After that, the abbot asks William to leave the abbey, claiming that he will try to solve the murder on his own. It is also implied that the abbot knew who the killer was but that he refused to reveal the truth.
William insists that they must solve the murder after the abbot left and he convinces Adso to enter the library one more time and try to find the finis Africae. William discovers the meaning of a riddle left being by Venantius and he and Adso enter the finis Africae.
Inside the room, they find Jorge who confesses that he killed the abbot by trapping him into a secret passage and letting him there to suffocate. Jorge confesses that he was the one who orchestrated the deaths of every person who died in the last few days inside the abbey but that he doesn’t feel guilty since he feels as if he was just doing God’s work. The forbidden book contained Artitotel’s ‘’Poetics’’ which talked about laughter and its place in man’s life.
Jorge begins eating the pages of the forbidden book, pages which have been laced with poison, and after that he escapes the room and tries to trap William and Adso inside it. William and Adso escape and they pursue Jorge but during the struggle some books inside the library catch on fire. The monks are unsure what to do and they simply look at the fire. Benno plunges into the flames and he is never seen again. The other try to save the library but soon the fire spreads to the entire abbey and it burns to the ground.
William and Adso leave after three days of trying to save the abbey. William gave Adso the pair of glasses Nicholas made him when his first pair was stolen and then he returned to England where he died of the black plague. Adso return home and years later, he visits the site where the abbey once stood. Adso collets what books and fragments he finds in the ruins before returning home. Adso finishes his memoir by telling the reader that he continued to study those fragments, looking for a meaning in the ashes of the past.