Symbologist Robert Langdon is visiting Paris when he is summoned to the Louvre Museum to examine the curious circumstances surrounding the murder of the museum's curator, Jacques Saunière. Saunière's body is found posed in a peculiar way that suggests a cryptic message; a pentacle is drawn on his stomach in his own blood, and a series of codes are left beside his body. With the assistance of cryptologist Sophie Neveu, Saunière’s granddaughter, Langdon begins to decipher the riddles, discovering that Saunière was part of a secret society—the Priory of Sion, which has ties to historical figures such as Leonardo da Vinci. The pair find themselves embroiled in a web of secrecy related to the Holy Grail.

As Langdon and Neveu delve deeper into the mystery, they uncover a trail of symbols hidden in Da Vinci’s artworks, most notably the Mona Lisa. These clues point to an ancient truth protected by the Priory, which could potentially shake the foundations of Christianity. Pursued by a determined French police captain and a shadowy religious zealot, they must piece together fragments of history and legend, navigating through a landscape of hidden passages and churches in Paris and beyond. The chase becomes deadly as they close in on the secret that Saunière sacrificed his life to protect, which has been sought after for centuries by many, including the clandestine Opus Dei faction.