Sienna Brooks helps him escape and discover what had happened during those missing days by following clues involving Dante's Inferno. When an assassin shows up in the hospital and attempts to kill Langdon, Dr. Robert Langdon wakes up in a hospital in Florence, Italy with a head wound and no memory of the last few days. Joined by his old flame and Solomon's sister, Katherine, Langdon follows a trail of clues that leads him deep into the secrets of the Masons, whose members include Solomon's tattooed kidnapper. Robert Langdon arrives in Washington DC at the behest of his mentor, Freemason Peter Solomon, only to find that he's been kidnapped. Joined by Saunière's granddaughter, Sophie Neveu, he seeks to prove his innocence by following a trail left by the secret organization Saunière was part of and discovers a shocking secret dating back to the time of Christ. When the curator of the Louvre, Jacques Saunière, is murdered, circumstantial evidence leaves Robert Langdon as the prime suspect. Arriving during the election of the new Pope, the two follow a secret trail to find the group's lair and stop them. Meeting Vetra's daughter, Vittoria, Langdon learns that the Illuminati, an ancient anti-religious sect, plan to destroy Vatican City with an antimatter canister invented by Vetra refashioned as a weapon. When physicist Leonardo Vetra is murdered and branded with the word Illuminati, his boss, CERN director Maximilian Kohler, asks Harvard symbologist Robert Langdon for help. Brown states on his website that his books are not anti-Christian, and that he is on a 'constant spiritual journey' himself. Brown's novels that feature the lead character also include historical themes and Christianity as motifs, and as a result have generated controversy. Langdon is portrayed as a Harvard University professor of religious iconology and symbology, a fictional field related to the study of historic symbols, which is not methodologically connected to the actual discipline of semiotics.

The Robert Langdon book series is named after Robert Langdon, the protagonist of the novels by American author Dan Brown. Novel series by Dan Brown Box set of the first four books in the series with their British covers

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