This novel starts off with a bang—literally.
Jacques Saunière, the respected curator of the Louvre museum in Paris, is viciously shot by an albino monk looking for a certain mysterious something…something only Saunière and three other men (that have already been offed) could direct him to.
Realizing the secret he and those other men vowed to protect is about to die with him, Saunière scrambles to leave behind a message that can only be understood by very specific people…
(Seriously, with an opening like this it's no wonder that The Da Vinci Code dominated the bestseller list for two. freaking. years.) (Source)
Robert Langdon, a humble but groundbreaking professor of Symbology, gets summoned to a gruesome murder scene in the middle of the night because the DCPJ (basically the French version of the FBI) need his expert opinion. Upon arriving, Langdon is stunned to realize the victim is Jacques Saunière—coincidentally, the very man he was supposed to meet for drinks earlier that night.
Dum dum dummm.
Captain Bezu Fache of the DCPJ likes Langdon as the prime suspect in Saunière's death, and all he needs is an inadvertent confession at the scene of the crime. He's thwarted in getting one, though, because Sophie Neveu (Saunière's estranged granddaughter and code-breaker extraordinaire) interrupts the process and cleverly warns Langdon that he's in serious trouble.
The two of them manage to escape the Louvre without being captured by the police…but not before they discover a series of clues left behind by her grandfather.
From there, Sophie and Langdon are led on a wild quest to find the keystone—a mysterious object that will ultimately lead to the Holy Grail. Turns out Saunière had been the head of a secret society known as the Priory of Sion (say that five times fast) who worship the sacred feminine and protect the secret of the Holy Grail's true identity and location.
You see, the holy grail's not a wooden cup like Indy Jones found: it's actually the bones of Mary Magdalene, and documents that prove she had been the mother of Jesus's children. (Whoa. That changes everything.) Now it's up to Sophie and Langdon to unlock his clues in order to discover the secret he's worked so hard to protect.
Along the way, while dodging capture by the DCPJ and attacks from the albino monk Silas, they enlist the help of the Grail expert Sir Leigh Teabing. Together they decipher Saunière's riddles and seek out answers all over London on a scavenger hunt to beat all scavenger hunts.
As things develop, it turns out this entire charade has been orchestrated by a shadowy figure known as the Teacher. (Sheesh, that's an ominous name.) He's the one who ordered Silas to kill Saunière, and he's been pulling the strings with Silas's mentor—the unfortunate Bishop Aringarosa—as well, who's a man who is desperate to save his conservative Catholic sect Opus Dei from extinction.
In a twist no one sees coming, we discover—just as Langdon deciphers the final clue—that Teabing is the dastardly Teacher. With the timing of a Swiss watch, Captain Bezu Fache manages to save the day. He'd realized he had the wrong man thanks to a confession from the sheepish Bishop (who'd realized he and Silas had been played), and manages to track down and arrest Teabing just in time.
With their names cleared of any crimes, and the final clue solved, Langdon and Sophie head to Rosslyn Chapel in Scotland. There they'll supposedly find the Holy Grail, according to Saunière's last riddle. Instead, though, they find Sophie's long-lost grandmother and little brother, who also happen to be direct descendants of Jesus Christ and Mary Magdalene.
Sophie's happy to finally have family (after losing her grandfather, she thought she was totally alone), but Langdon is pretty bummed that he didn't end up finding the Grail.
Thankfully, Langdon pieces together Saunière's final clue in a different way, and is struck by the realization that the Holy Grail is actually hidden beneath the Louvre itself.