On the order of Abbot Abbone, and while the dead continue 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 He 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 heretics Dolciniani exterminating the family of Anna, daughter of Dolcino and Margherita.