In Montefiore, Italy in the early 16th century, a nobleman named Don Vicente de Nevada lives on a small estatewith his seven-year-old daughter, Bianca, and a small staff, two of whom are Primavera, an earthy cook and a friar Fra Ludovico. The eponymous mirror was fashioned by dwarves and left in the pond to temper, where, at the beginning of the novel, it is found by de Nevada.
Life is good for the family until the day the duchessLucrezia Borgia and her brother, Cesare, decadent children of a pope, come to visit. Cesare sends Vincente on a quest for a holy relic. While he is gone, Bianca becomes a young woman and Lucrezia becomes jealous of the girl's beauty and stealing Cesare's attention from Lucrezia. Eventually she hires a hunter to kill Bianca, who instead helps her escape from Lucrezia. The girl escapes, and runs into seven dwarfs, who are looking for the eighth dwarf and their mirror. The eighth dwarf is accompanying and protecting de Nevada on his travels.
When the mirror reveals to the duchess that her plan has failed, she takes it into her own hands to kill Bianca. When she eventually succeeds, Bianca is placed in a coffin, with the now-liberated mirror allowing passers-by to view her beauty. Eventually, she is awakened subsequent to a kiss from the very hunter who helped her escape. The device by which the kiss cures her of mercury poisoning is left unexplained by the author.