Prague, 1968. A young translator receives an old French book of memoirs of the Reverend Adso from Melk. The title page of the book states that it is a translation of a Latin text written in the end of XIV century that also was a reproduction of an even more ancient manuscript of the XIV century. The translator starts the investigation, unfolding the thread from the newest copy to the original memoirs, but finds no traces of Adso himself. A strange book can be the last copy left intact or just a skilled fake. Moreover, later it also disappears, leaving the translator even more uncertain about this retelling of a strange Medieval story.
The elderly Benedictine monk named Adso recalls the events he happened to witness and participate in during the 1327. At that time the whole Europe is shaken by the political and religious discords. Emperor Louis opposes the Pope John XXII. At the same time the Pope is oppressing the order of Franciscans. The monks join the Emperor to fight back and their order becomes a significant force in the political game.
In all this mess, Adso, a young novice monk at that time, accompanies the English Franciscan monk, Wilhelm Baskerville. They travel through the cities and largest monasteries of Italy. Wilhelm is a philosopher and theologian and also a naturalist, well-known for his powerful analytical mind. He is a friend of William Okkam and a student of Roger Bacon that already says a lot about him. Now Wilhelm is working on the Emperor’s task to prepare and hold a meeting between the Imperial delegation consisting of the Franciscans and the representatives of the Pope’s Curia. Wilhelm and Adso arrive in the abbey that is going to host the meeting a few day before the rest of the participants. The meeting shall be in the form of a theological debate where the main topic is the poverty of Christ and the church, but the deeper goal is to find out the positions of both parties and the possibility of the future meeting of the head of Franciscans and the Pope in Avignon.
Even before entering the abbey, Wilhelm surprises the monks he meets with his precise deductive conclusions. They go out to find a missing horse and Wilhelm, not even seeing that horse, helps them find it using only his logic. This case becomes known to the Abbott and he immediately asks Wilhelm to help him investigate one strange death of a young monk Adelmo. His body was found at the bottom of the cliff. Perhaps, he was thrown from the window of the building towering over the cliff. The Abbott hints that he knows the true circumstances of Adelmo’s death, but he is bound by his oath and can’t disclose the story of confession. Therefore the truth shall be discovered and vocalized by another person.
Wilhelm is granted permission to ask every member of the monastery without exception and visit and examine every places of any building, except for the famous monastery library. The library is the largest in all the Christian world and so rich that it is even compared with the burnt library of Alexandria. It is located on the upper floor of the building Adelmo fell from. Only the librarian and his assistant have the access to it and know the plan of the library, deliberately built as a labyrinth. Without them no one is able to find the right book on the endless shelves. All the other monks who work with the books: copyists, translators and restorers of the old manuscripts, are allowed only to one special room. The monks from all the Europe try to get there to work and make a copy for themselves. The librarian alone decides when and how to provide the book to the one who claimed it and whether to provide it at all - because there are a lot of pagan or heretical works that can be dangerous for unprepared minds.
In this room Wilhelm and Adso meets the librarian himself named Malachi and his assistant Berengar.Other people working here are Venantius - a young translator from Greek and a huge fan of Aristotle - and Benno, a student of rhetoric. The constant visitor of the room is an old and grumpy blind monk named Jorge. He usually appears to scold the younger ones for laughing or chatting. He lives awaiting the Last Judgement and is sure that nowadays everything is horrible and corrupted.
Examining the abbey, Wilhelm comes to the conclusion that Adelmo probably wasn’t murdered but committed suicide by jumping from the wall of the abbey. His body was later transferred to the place it was found by a landslide. But in that very night the body of Venantius is found in the barrel with the fresh blood of slaughtered pigs. Wilhelm investigates the site and discovers that the young monk was killed somewhere else, most probably near the library, and was thrown into the barrel when he was already dead. But the body is clear, without any wounds, injuries or signs that the monk was fighting someone back.
Seeing that Benno is too excited and Berengar is outright horrified, Wilhelm immediately interrogates both. Berengar confesses that he was probably the last who saw Adelmo before he died. Pale as a dead man, Adelmo told him he was cursed and doomed, that he will go to Hell for eternal suffering and he described this suffering and torments very vividly and plausibly. Benno adds that two days before Adelmo’s death they had a strange dispute in the scriptorium room. Adelmo’s illustrations in the rewritten books were quite funny and some of the monks found them inappropriate, but Adelmo replied that holy truth is better understood while it appeals to something earthly and common than to the highly metaphors that are hard to grasp. Excited by the dispute Berengar accidentally tells, though vaguely, about one of the greatest secrets of the library somehow connected with Africa. Benno saw only one suitable position in the library catalog named finis Africae, but when he asked the book the librarian replied that it has been lost. Benno also tells that he followed Berengar after the dispute and saw something horrible.
After the interrogations Wilhelm gets a new picture. It seems that Adelmo, in exchange for a certain things, maybe a forbidden book from the library, paid Berengar with sex. But after it Adelmo couldn’t live with it and went to the blind Jorge to confess. Instead of forgiveness Jorge promised him all the sufferings in Hell, Adelmo later recited to Berengar. The monks in the abbey are all agitated, constantly thinking both about knowledge they have and the dangers and punishments for misusing this knowledge. Such a sensitive young man as Adelmo was so horrified by Jorge’s words that he considered himself already in Hell and killed himself in despair.
Wilhelm tries to examine the books on the Venantius table in the scriptorium room. But he is constantly distracted: first by Jorge then by Benno. Wilhelm ask Malachi to call someone to guard the table of the deceased. He discovers a secret passage used by librarian and uses this at night to return to the scriptorium with Adso. Among the Venantius’ papers they see a strange script with undecipherable phrases and strange signs, but the book Wilhelm saw on the table during the day is absent. Suddenly they hear a sound - they are not alone in the scriptorium. Wilhelm chases the intruder. The mysterious person drops a book but they manage to pick it up and run away before Wilhelm catches them.
At night the fear and superstitions guard the library much better than the locks. Many monks believe that in the darkness there are horrible creatures and souls of the previous librarians wandering along the shelves. Wilhelm is quite sceptical about that and uses the secret passage to examine it. To Adso’s horror the superstitions seem to be true - but these are only mirrors and the delusion-inducing evaporations they inhale in the labyrinth. What really makes it dangerous is their inability to find the way out. Wilhelm and Adso find the door just because of sheer luck. And then the terrified Abbott tells them that Berengar also disappeared…
The dead body of the librarian’s assistant is found only after a day, in the bathroom near the hospital. The local healer and herbalist named Severinus works with Wilhelm as a coroner. Severinus draws his attention to the fact that Berengar has some strange substance on his fingertips. The herbalist adds that he saw the same substance on Venantius’ fingers while his body was cleared of pigs’ blood. Moreover, Berengar has the black tongue - it means he was poisoned before drowning. Severinus tells that he used to have a strong poison that he never used and didn’t know its properties. That poison was lost under the strange circumstances. The only people knowing about the poison are Malachi, the Abbott and Berengar himself.
In the meantime the embassies finally reach the abbey. One member of the delegation from the Pope is the notorious inquisitor Bernard Gui. Wilhelm openly despises Bernard himself and his methods of work. Bernard learns about the mysterious murders and immediately claims that now he will investigate the case himself. He is sure that the Devil is involved here.
Wilhelm and Adso enter the library again to make a map of the labyrinth. It appears that every room of it is marked by the letters. Passing through them makes the codewords and the names of the countries. They find the finis Africae - the secret room, disguised and thoroughly closed - but they can’t enter it. While they are working in the library, Bernard Gui captures and condemns the healer’s assistant and a village girl. The assistant helped the girl to enter the monastery and have sex with the healer for food. Adso also met her before (and also was seduced by her beauty). Now the girl will be burnt on a stake as a witch, guilty of all the mysterious murders.
The polite and civilized dispute between the Franciscans and the Pope’s representatives turns into a brutal fight. Wilhelm stays away of it and Severinus tells him that he has just found a strange book in his laboratory. Their conversation is heard by blind Jorge. The dispute continues after the fight and Wilhelm returns to his duty as its moderator. But the discussion is ended by another horrible news: the healer is found dead in his laboratory and the murderer is already captured.
Severinus’ skull is cracked by the heavy metal globe he had on his desk. Wilhelm searches for the familiar marks on the body’s fingertips but the healer is in thick leather gloves he used while working with dangerous substances. The main suspect is Remigio - the cellarer who discovered the body. He tries to explain everything, saying that the healer has been already dead when he entered the laboratory, but in vain. Benno says to Wilhelm that he was one of the first who was in the lab and then he looked attentively. He is sure that Malachi was already here, waiting behind the curtain and then joined the rest of the crowd unnoticed in all the mess.
Wilhelm is sure that: the book is too big and heavy to be taken from the lab secretly and if the murderer is Malachi, then the book shall still be in the lab. Wilhelm and Adso start searching, but they forget that sometimes the different manuscripts are gathered under the one cover. As a result they go past the book they consider insignificant, but Benno notices the book instead and takes it.
Bernard Gui condemns the cellarer. He accuses Remigio of being a heretic and forces him to take all the guilt for every murder in the abbey. The inquisitor isn’t interested who is the real murderer. What is really important for him is that the accused Remigio is connected with Franciscans. This evidence is enough to abort the meeting, that, as it seems, is the main his goal here.
Wilhelm understands that Benno now has the book and demands it back. Benno replies that he returned it to Malachi without even starting reading. Instead Malachi offered him to replace the deceased Berengar as his assistant. Several hours ago, during the sermon, Malachi falls on the floor in agony and dies. He also has black tongue and the familiar marks on his fingertips.
The Abbott bitterly says Wilhelm that he expected more of him. He demands both Wilhelm and Adso to leave the abbey tomorrow. Wilhelm replies that he knows about Berengar’s homosexuality that caused the first suicide but it isn’t the case of everything that happened later. The monks that died are connected with one same trace: they all know about the finis Africae. The Abbott looks like Wilhelm’s words hinted at something for him, but remains silent about it. He still insists on Wilhelm leaving as soon as possible, because now he wants to continue the investigation personally, considering it his own responsibility.
But Wilhelm doesn’t want to give up: he is sure that he has almost solved the case. Adso accidentally helps him to decipher the Ventianus script. It is the key to finis Africae. On the last night they enter the library again and open the secret room. The blind monk Jorge waits for them inside.
Wilhelm expected to meet him there. The secrets and suddenly finished conversations of the monks, the marks in the catalog and some other evidences helped him to learn that Jorge was once a librarian himself. When his sight started to weaken he taught his first apprentice, then Malachi. But no one of them dared to work without asking Jorge of every their decision. Even the Abbott himself was dependant on Jorge, because he got his position thanks to the old monk. For forty years the blind monk is a true master of the abbey. He is sure that some of the manuscripts hidden there must be hidden forever from any mortal eyes. When Berengar, in his desire to sleep with Adelmo, stole one of these, Jorge did what he had to do to return it back. The book worth killing is the second part of Poetics by Aristotle that was considered to be lost. The book is dedicated to the nature of funny things in the arts, rhetorics, and persuasion. That’s why Adelmo, fascinated by the idea of connecting funny and sacred, was ready to bargain his very soul to get this book.
But Jorge is also ready to do anything just to erase any trace of the book’s existence. He believes that if the laughter and fun are legalized by Aristotle’s authority, the entire medieval hierarchy and the culture of solemn thinking of sins and punishment will be destroyed. With it will fall everything good and right that exists in people and they will lose their very souls in the vulgar and materialistic secular goals and desires.
Jorge admits that he waited for Wilhelm to find him from the very beginning of the investigation. He silently watched as the Englishman unfolded the evidences and traces coming closer to him. Jorge gives the book to Wilhelm, offering him to read it. But the Franciscan says that he indeed understands everything and tells the whole story to shocked Jorge . Long ago, hearing that someone in scriptorium was interested in finis Africae, Jorge - at that time still a librarian - stole the poison from Severinus, but saved it for later. But when Berengar, trying to buy Adelmo, attempted to take the book for the first time, Jorge, already blind, went to the secret room and soaked the pages of the book with poison.
Adelmo, tormented by shame, didn’t open it, but, after listening to Jorge’s threats, he told everything to Venantius, hoping that his friend would relieve him of fear and despair. Later Venantius took the book himself, eager to read and copy another work of the great Aristotle, but to open the sticky pages he had to lick his fingers. He died after reading only a few of them.
Berengar found the body and, fearing that the investigation would disclose everything that has done to Adelmo, hid the body in the barrel with blood. Wilhelm chased Berengar when they entered the library for the first time. Berengar was interested in the book too, he took it to the hospital, sure that no one would search for the book there. When the poison started to consume him, he fell into the water in the bathroom, trying to drink as much as he could to soothe the pain in his stomach.
Severinus found the book and told Wilhelm about it. Jorge sent Malachi to kill the healer - familiar with his own potion, Severinus put on gloves before reading. Malachi killed the healer with the metal globe but later died himself, deciding to read the mysterious book he had to kill for. The last one in the row shall be the Abbott.
Jorge has no options than to agree. The Abbott indeed came to him demanding explanations. Moreover, he ordered to open finis Africae and put an end to the conspiracy that gives Jorge so much power. Now the Abbott is slowly dying in the hidden oubliette Jorge threw him into and then broke the mechanism that opened the door.
Wilhelm replies that everyone died in vain. The book is found and Jorge didn’t manage to kill him. Jorge will now be punished for everything he has done. But the blind monk is ready to die for his ideas too: he tears the pages from the book and eats them. When Wilhelm tries to stop him, he runs away. Even blind, Jorge knows the labyrinth of the library much better than Wilhelm who has to use the lamp to see his way. Wilhelm and Adso catch Jorge, but the monk fights back and breaks their lamp, throwing it away. The burning oil leaks to the books and the huge fire starts immediately. Wilhelm and Adso run away to bring water, but it is too late. All the monks altogether can’t stop the fire, it burns the library, then the church and all the other buildings of the abbey.
Adso witnesses as the rich and prosperous abbey that was a sacred place for everyone who praised knowledge, burns to ashes. The abbey burns for three days. Then the monks, gathering the scarce possessions they managed to save from the fire, leave the ruins of the place that was their home and then was cursed by God.