After another outrageous event occurs, William's investigation leads him and his apprentice Adso to the doors of the mysterious library, where everyone but the librarians are forbidden to enter. Meanwhile, the inquisitor Bernardo Gui, Pope John's ambassador, brings the wrath of his fanaticism to anyone who cross his path.

While William asks for a magnifying glass to be able to proceed with his investigation, Adso meets the mysterious Occitan girl and huntress Anna pursues Bernardo Gui's entourage to avenge her family.

After a new tragic event occurs, William and Adso increase their efforts to decipher the mysterious structure of the labyrinthine library, although Adso is more interested in spending time with the Occitan girl.

Anna is saved at the last minute. Bernardo Gui's armed entourage arrives at the abbey. After a copious feast, an insidious monk poisons his ears by reporting the presence inside the sacred walls of some former Dolcinian heretics.

Anna gets better. The Occitan girl gets into trouble. Meanwhile, the debate continues, but is interrupted by another tragic event.

Gui uses Salvatore to get to Remigio and put him on trial. William gives his thoughts on both the show trial and the poverty debate between the Franciscans and the Church. Anna tries to rescue her friends, and another mysterious death occurs.

With the debate over, Gui condemns Remigio and the girl to be burnt at the stake. William and Adso try to solve the library's puzzle and catch the killer before it's too late, and Anna makes a final desperate move.