As Sophie races to the Mona Lisa she passes near her grandfather's body, and she's suddenly seized with remorse about all the years in which he reached out to her, only to receive silence in return.
She needs a black light to read any message he might've left her, so she's compelled to go right to the crime scene to pilfer one from there.
When she gets to the Mona Lisa, Langdon's waiting for her.
Langdon anxiously asks Sophie to recall whether Saunière had used the initials "P.S." anywhere else—like a monogram, or on a personal item at all.
He did indeed. (Good call, Langdon.)
When she was snooping around the house trying to discover her birthday present at the age of nine, she'd accidentally discovered a large golden key on the end of a chain.
It was unlike any other key she'd ever seen, with a weird cross, and the initials "P.S." intertwined with a fleur-de-lis.
Sophie remembers her grandfather discovering her with the key, and solemnly admonishing her to respect other people's privacy.
The next day, her birthday, her grandfather didn't even wish her a happy one. She was totally bummed that she was in trouble, but when she went to bed her grandfather had left her a birthday scavenger hunt, with riddles for clues.
At the end of the hunt, she ended up back in her bedroom, where she found a beautiful red bicycle with a ribbon on the handlebars, and a smiling grandfather.
Later, he'd tell her that the key was very important, and it opened a box where he kept many important secrets.
Sophie hated secrets, so he made her a deal: if she could keep it a secret, and never talk about the key ever again to anyone, someday he'd give it to her.
And that's when he gave her the nickname Princess Sophie, because now it was her initials on the key.
Sophie exits her reverie (man, these two daydream a lot, don't they?) and reluctantly tells Langdon about the key with the initials on it.
Langdon asks if it also had a fleur-de-lis.
Sophie's shocked that he'd know that. Langdon reveals that he has a new theory: that her grandfather was a member of a very old secret society, the Priory of Sion.
Sophie's horrified, because the thing that caused their falling out was proof of his involvement with a secret society.
Langdon admits that studying symbols of secret societies is something of a specialty of his.
He gives Sophie a brief rundown of the Priory's history, including Leonardo da Vinci's role as Head Master, and their focus on being the pagan goddess worship cult. (Sorry: they had "reverence for the sacred feminine".)
The Priory were also keepers of an immeasurably powerful secret.
It's finally starting to click why Saunière wanted Sophie to find Langdon: he could help her understand everything.
Now Langdon is having a daydream. He's lost in the possibilities of all the secrets about to be revealed…
And a few miles away, Fache is just realizing that it was a bar of soap that escaped on the tarp-covered truck.