Jacques Saunière, a Louvre curator, is pursued through the Grand Gallery by an albino Catholic monk named Silas, who demands the location of the Priory's "keystone" to find and destroy the Holy Grail. Saunière gives him a false lead and is murdered. The police find his body posed like Da Vinci's Vitruvian Man. Police captain Bezu Fache has his lieutenant, Jérôme Collet, summon American symbologist Robert Langdon, who is in Paris for a lecture on the interpretation of symbols, to examine Saunière's body.
Langdon is shown the body and a secret message, readable only by blacklight. It contains an out-of-order Fibonacci sequence. Sophie Neveu, a police cryptographer and Saunière's granddaughter, tells Langdon that Fache planted a tracker on him after finding the words, "P.S. Find Robert Langdon" at the end of Saunière's secret message. Fache believes Langdon murdered Saunière. Sophie throws away the tracker, distracting the police while they sneak around the Louvre, finding more clues in Leonardo da Vinci's works. Langdon deduces that Saunière was the grand master of the Priory of Sion.
Silas works for an anonymous person referred to as "the Teacher", along with members of Opus Dei, led by Bishop Aringarosa. Langdon and Sophie travel to the Depository Bank of Zurich and access Saunière's safe deposit box by using the Fibonacci sequence. Inside is a cryptex, a cylindrical container that contains a message on papyrus. It can only be opened without destroying the contents by turning dials to spell a code word. As police arrive, bank manager Andre Vernet helps Langdon and Sophie escape, then attempts to steal the cryptex and murder them. Langdon and Sophie escape with the cryptex.
They visit Langdon's friend, Sir Leigh Teabing, a Holy Grail expert. Teabing claims the Grail is not a cup but instead is Mary Magdalene. He says she was not a prostitute but the wife of Jesus Christ. Teabing argues that Mary was pregnant during His crucifixion, and the Priory formed to protect their descendants. The Opus Dei have been trying to destroy the Grail to preserve the credibility of the Vatican. Later, Silas breaks into Teabing's house, but Teabing, who uses crutches, uses one to disable him. The group escapes to London via Teabing's private plane, along with his butler, Remy Jean. Their misinterpretation of the clue that dropped from the cryptex leads them to the Temple Church, where they find nothing. Remy, who claims to be the Teacher, frees Silas. Remy takes Teabing hostage, dumping him in the car trunk, and taking Silas to hide out in an Opus Dei safe house. Teabing, who is revealed as the Teacher, later poisons Remy and sends the police after Silas. Police shoot Silas after accidentally wounding Aringarosa, who is promptly arrested by Fache, who resents being used to hunt Langdon.
Teabing, who wants to bring down the Church for centuries of persecution and deceit, confronts Langdon and Sophie. Now understanding the true meaning behind the clue to unlock the cryptex, the trio goes to Westminster Abbey to the tomb of Isaac Newton, a former grand master of the Priory. Teabing demands that the pair open the cryptex. Langdon tries and seemingly fails before suddenly tossing the cryptex into the air. Teabing dives for it, catches it, but the vial breaks, and the papyrus is thought destroyed. The police arrive to arrest Teabing, who realizes Langdon must have solved the cryptex's code and removed the papyrus before throwing it. The code is revealed to be "APPLE", after the apocryphal myth of the apple which led Newton to discover his law of universal gravitation. The clue inside the cryptex, which tells of the Grail hiding "'neath the rose," leads Langdon and Sophie to Rosslyn Chapel in Scotland.
Inside the chapel, they discover a secret room where Magdalene's tomb has been removed. Langdon, after searching through documents, realizes that Sophie's family died in a car crash, that Saunière was not her grandfather but her protector, and that she is the last descendant of Jesus Christ. The two are greeted by several members of the Priory, including Sophie's grandmother, who promises to protect her. Langdon and Sophie part ways, the former returning to Paris. While shaving, he cuts himself and has an epiphany when his blood curves down the sink, reminding him of the Rose Line. Realizing the true meaning of the cryptex clue, he follows the line to the Louvre, concluding the Holy Grail is hidden below the Pyramide Inversée. Langdon kneels atop it and the sarcophagus of Mary Magdalene is seen in a secret underground chamber.
Tom Hanks as Robert Langdon
Audrey Tautou as Sophie Neveu
Ian McKellen as Sir Leigh Teabing
Alfred Molina as Bishop Aringarosa
Jürgen Prochnow as André Vernet
Jean Reno as Police Captain Bezu Fache
Paul Bettany as Silas
Étienne Chicot as Lieutenant Jérôme Collet
Jean-Yves Berteloot as Remy Jean
Jean-Pierre Marielle as Jacques Saunière
Charlotte Graham as Mary Magdalene
Hugh Mitchell as young Silas
Seth Gabel as Michael the Cleric
Marie-Françoise Audollent as Sister Sandrine
Francesco Carnelutti as Prefect
Rita Davies as Elegant Woman at Rosslyn
Denis Podalydès as Flight Controller
Author Dan Brown and his wife make cameos (forefront) in the first scene of the book signing scene.
The Templar Revelation authors Lynn Picknett and Clive Prince make a brief appearance as passengers on a bus.