Langdon and Sophie run to her tiny SmartCar parked on the plaza, fleeing the police who're in hot pursuit.
Except…the cops aren't following them. They're all assembling at the Louvre.
Langdon admires Saunière's sense of humor by choosing Madonna of the Rocks as his last message's hiding place.
Sophie tells him she'll show him what her grandfather left her when they're safely inside the embassy, but hint hint it's the key.
She knows the key is for something very sophisticated—it's a "laser-tooled varying matrix", but she has no idea what it unlocks.
Then she flashes back to that fateful day ten years ago that scarred her so severely. She'd come home from graduate school early as a surprise, and drove out to their country chateau in Normandy.
When she got there, there were tons of fancy cars parked at the house.
Although there was every sign of a large party going on, the house itself was quiet. Too quiet. And apparently totally empty.
She let herself in using a spare key and searched the abandoned chateau.
Then she heard strange chanting coming from the basement—a basement she didn't know even existed.
She found a hidden doorway, and entered the secret basement. There she found everyone in masks, dressed in white gowns and black tunics. They were chanting reverently towards something in the middle of the circle. This "something" disgusted her.
Sophie fled the house after leaving a note that said, "I was there. Don't try to find me." And they hadn't spoken since.
She's awoken from her reverie by Langdon, who's trying to warn her about the flock of cops in front of the embassy.
When she suddenly stops and turns around, the cops start to follow. Oh merde.