The Name of the Rose begins with a prologue by an unknown narrator, who explains how he found a transcription of a medieval manuscript containing the account of Adso of Melk, a fourteenth-century German monk. Although the narrator expresses doubts about the authenticity of the text and the veracity of the incredible story it tells, he has decided nonetheless to translate and publish it in Italian. He explains that the manuscript is divided into seven days, and each day into eight sections corresponding to the times of the day at which the monks prayed (matins, lauds, prime, terce, sext, nones, vespers, and compline). The second prologue introduces us to Adso of Melk, a Benedictine monk who is writing this manuscript at the end of his life. The action then shifts to the late 1320s, when Adso was a novice of about eighteen years old, traveling Italy in the service of William of Baskerville, an English Franciscan and former inquisitor.
On the first day, William and Adso visit an unnamed abbey somewhere in northern Italy, where William plans to attend a theological disputation on the contentious problems of whether Christ had been poor and whether the pope or the Holy Roman Emperor should hold political authority in Europe. William reveals his powerful skills of deduction when he locates a runaway horse on the scant evidence of prints in the snow. The abbot, Ado of Fossanova, tells William about a strange and mysterious event: a monk named Adelmo of Otranto, who worked as an illuminator in the scriptorium, recently fell to his death from the tallest tower of the abbey. Ado asks William to discreetly investigate before the envoys arrive, and William agrees. While Adso admires the beauty of the abbey, William debates with Ubertino of Casale, a Franciscan monk who has fled the order because of political persecution. Adso and William also meet other monks: Remigio of Varagine, the cellarer; Salvatore of Montferrat, a monk who speaks an odd combination of several different languages; Severinus of Sankt Wendel, the herbalist; Malachi of Hildesheim, the librarian; Berengar of Arundel, his assistant; Venantius of Salvemec, a translator; Benno of Uppsala, a rhetorician; Jorge of Burgos, a blind monk; and many others. Malachi allows Adso and William to visit the scriptorium but explains that only the librarian is permitted to enter the labyrinthine library. William and Jorge argue about the merits and permissibility of laughter.
Venantius is found dead in a barrel of pig’s blood on the morning of the second day, although it appears that he didn’t die from drowning. Benno tells William and Adso about Adelmo and Venantius’s request for a forbidden book in a room in the library called the “finis Africae,” and alludes to an illicit relationship between Berengar and the deceased Adelmo. Later, an obviously guilty Berengar tells William and Adso about his encounter with the “ghost” of Adelmo in the cemetery. Benno explains that Berengar had persuaded Adelmo to have sex with him in exchange for access to the mysterious book. William and Adso decide to break into the scriptorium to investigate, where they find a manuscript written in code on Venantius’s desk. While William is distracted, someone steals his glasses to prevent him from being able to read the manuscript. In the library, William and Adso get lost in the labyrinth: there are dozens of rooms, and they don’t understand the words above each doorway.
On the third day, Berengar disappears, leaving only a blood-stained cloth behind in his cell. Wandering through the abbey, Adso reflects on the monks’ thirst for the knowledge contained in books and talks with Salvatore, who tells him about his life as a follower of the Franciscan heretic Fra Dolcino. Disturbed by what he has heard, Adso asks William and Ubertino to explain the story of Dolcino and the factional divisions within the Franciscans. Eventually, William mentions that he has deciphered a cryptic message from Venantius’s manuscript: “The hand over the idol works on the first and the seventh of the four.” He also has some theories about how the rooms in the library might be organized, after examining the building from the outside. That night, Adso encounters a girl from the village in the abbey kitchen; they have sex, but she flees the next morning, leaving an ox heart behind. Adso confesses his sin to William, who is compassionate. In the kitchen, they find the body of Berengar drowned in a tub.
On the fourth day, Severinus observes that Berengar’s tongue is black. Salvatore explains the presence of the girl in the kitchen, admitting that he procures women from the village for Remigio, who offers them food in exchange for sex. Severinus finds the stolen glasses in Berengar’s pocket and returns them to William, who is then able to decipher the Greek portion of Venantius’s manuscript—but the words seem nonsensical. William eventually theorizes that the Greek words refer to the Book of Revelation, and that the crimes are following a sequence according to the Apocalypse. Meanwhile, the Franciscans and the pope’s envoys begin to arrive at the abbey. William and Adso visit the labyrinth again, discovering that the labels above the doors refer to regions of the world, and that books are organized according to their country of origin. They locate the finis Africae—a walled-up room concealed behind a mirror—but are unable to figure out how to enter. When they return from the library, they find that Salvatore and the girl Adso met the night before have been arrested, since Salvatore had been using superstitious rituals to try to cast a love spell on her. The girl is accused of witchcraft and condemned to death; Adso is devastated but unable to help her.
At the theological disputation on the fifth day, the envoys debate whether or not Christ owned property and how much political influence the pope should have in Europe. William argues that the church should confine its influence to the religious sphere. Severinus tells William that he has found a strange book among his library, but is murdered before he can explain further; William and Adso realize too late that the book in question is an Arabic manuscript, but when they return to the infirmary someone has stolen it. Under the interrogation of Bernard Gui, an inquisitor, Remigio falsely confesses to the murders. Benno admits that he stole the book and returned it to Malachi. That evening, Jorge gives a sermon predicting that the Last Judgement is at hand and reproaching those who seek to know more than God intended.
On the sixth day, Malachi dies—his fingers, too, are blackened. This proves decisively that Remigio is not the murderer, although Bernard had declared the case closed. Adso falls asleep in church and dreams of the Coena Cypriani (the “Feast of Cyprian,” a Latin carnival comedy), which reminds William to check the library catalogue: and sure enough, the mysterious book contains a copy of the Coena along with several other texts. Examining the handwriting in the catalogue, William comes to the conclusion that there is a nameless other librarian not mentioned in the official record. When he brings his conclusions to Ado and asks to see the mysterious book, the abbot tells him to stop investigating. In defiance of the abbot’s orders, William and Adso return to the library and discover the secret entrance to the finis Africae.
That evening, on the seventh and final day, William and Adso enter the finis Africae. There they find Jorge, who admits that he has controlled the library for decades; Malachi takes orders from him. He also confesses to the murders: he poisoned the pages of the book, thus killing Venantius, Berengar, and Malachi (explaining their blackened fingers and tongues), and incited Malachi to murder Severinus. There was no scriptural pattern to the murders. Instead, he has done all this to keep the secret of the finis Africae: the room holds the only surviving copy of the lost second book of Aristotle’s Poetics, a treatise on comedy that Jorge thinks would undermine religion and overturn the order of society. Rather than allow William to take the book, he eats the poisoned pages himself. In the ensuing struggle, Jorge knocks over a lamp and sets the library aflame. The fire quickly gets out of control, burning down not only the library but also the entire abbey.
In the wake of the destruction, the monks disperse and Adso returns home to Melk. Adso reports that William gave him his glasses as a parting gift, and died shortly afterwards during an outbreak of the Black Death. Years later, a much older Adso returns to the site where the abbey had once stood to collect what remains of the library: a few scattered leaves and fragments of parchment. As he finishes writing his story, he prepares himself for his own death.