The story opens with Silas tightening a spiked metal cilice around his leg before taking a handgun and leaving. Robert Langdon, a Harvard professor of symbology in Paris for a lecture, is then brought to the Louvre by Captain Bezu Fache after the murder of his friend and museum curator Jacques Saunière.

At the crime scene, Fache shows Langdon Saunière’s numeric cipher and the blacklight message, “O Draconian Devil! Oh Lame Saint!” Sophie Neveu of the cryptography department arrives and explains that the numbers are part of the Fibonacci sequence, though out of order. She later warns Langdon that he is in danger, and tells him the police have planted a GPS tracker on him.

Langdon and Neveu work out that the clues are anagrams, leading them to “Leonardo da Vinci” and “The Mona Lisa.” More blacklight clues send them to Saunière’s office and then to Saint-Sulpice, where Langdon learns that Sandrine is dead and that Silas was looking for something she refused to give him. He later finds a stone tablet in the crypt and discovers that Saunière was the current Grand Master of the Priory of Sion.

Neveu reaches Saunière’s chateau, evades Silas and the police, and follows clues to a key with the address of the Depository Bank of Zurich. She meets up with Langdon, and together they open Saunière’s deposit box and find a cryptex before heading to Château Villette.