Sophie remembers that her grandfather liked to create anagrams of famous paintings. When she was young, he took her to visit the Mona Lisa when the museum was closed. She did not think much of the painting at the time. She realizes that the Mona Lisa would be a good place for him to leave her a message, and that he would have been able to visit the painting before he died. Sophie decides to go back up the stairwell to find the painting. She tells Langdon to go to the embassy without her and gives him the keys to her car. As he walks away, Langdon wonders why Saunière told Sophie to find him. Sophie could easily have figured out the puzzle in the message without him. While thinking about the letters “P.S.” in the code, Langdon has a sudden realization. He starts running back to Sophie.
At the Church of Saint-Sulpice, Silas looks around the sanctuary and finds the Rose Line, a strip of brass on the north-south axis that is imbedded in the structure of the church. This line, a pagan sundial, was the zero longitude of the world before Greenwich, England, took that title. Silas has been told that the Priory keystone lies beneath the obelisk at the northern terminus of the line. He walks toward the obelisk. Meanwhile, Aringarosa arrives in Rome.
Sophie tries to see whether her grandfather left her any messages in invisible ink by the Mona Lisa. Langdon reappears, out of breath. He asks Sophie if the initials P.S. mean anything to her aside from Princesse Sophie. She says that once, when she was younger, she saw a strange key in her grandfather’s closet decorated with the initials P.S. and—as Langdon has already guessed—a fleur-de-lis. Saunière never explained what the key was for, but he said if she kept the secret, the key would one day be hers.
Langdon says that Saunière was a member of the Priory of Sion, an exclusive secret society involved in pagan goddess worship. The Priory has had many prominent members, among them Leonardo Da Vinci. It is known as the protector of a huge secret. Sophie thinks that this might explain the unthinkable scene she witnessed her grandfather taking part in. Meanwhile, Fache and his partners apprehend the truck and discover the bar of soap with the GPS tracker in it.
Silas kneels at the base of the obelisk. Each of his victims told him that the keystone was hidden there. He knocks on the tiled floor and discovers that there is a hollow opening under the ground. He prepares to break the floor tile. Sister Sandrine, spying on him from the balcony, prepares to do her duty as a sentry for the Brothers of Sion. She thinks the stranger standing at the base of the obelisk is a message from the dead Brothers telling her that something is wrong.
Fache calls the American Embassy and discovers that there was no message for Langdon. He backtracks through numbers on his cell phone and finds the number that Langdon called. When he realizes that it was Sophie Neveu’s number, he becomes angry. He punches in the access code.
The police investigators, Sophie, and Langdon have all been in the same room with Da Vinci’s Mona Lisa without realizing that the painting is central to discovering Saunière’s secret. The Mona Lisa has historically been associated with secrecy; Mona Lisa’s half-smile is famous for its ambiguity, and the sfumato style of painting, which produces a foggy effect, increases the sense of mystery. Many have speculated about the cause of Mona Lisa’s smile. Some, like the young Sophie, have failed to understand the painting’s fascination. Like the meaning of the Mona Lisa’s smile, the secret that Saunière knew seems to be hidden in plain sight.
Brown does not reveal the details of the terrible act Sophie witnessed her grandfather performing. At this point, it is impossible to know whether the act was as horrible as Sophie says, or whether there is some sort of Priory-related explanation for it. Brown has portrayed Sophie as a fairly open-minded person, which suggests that her interpretation of her grandfather’s behavior is probably accurate.
Silas, like Fache, has failed to see that the women around him are not necessarily just unthinking, silent witnesses. Sandrine, like Sophie, has been able to use men’s underestimation and her placement near the scene to exert influence on the action.
Modern communications present an interesting contrast to the ancient signs and symbols that preoccupy the book’s characters. Fache’s use of the cell phone as the way to break the code of Sophie’s betrayal has more in common with modern-day spy movies than with the ancient mysteries that the rest of the novel explores.