This should be readable regardless of whether you follow Supernatural. All you really need to know is that, in Supernatural terms, a hunter is a person who specifically hunts evil supernatural things. Mary is the mother of the two main characters; we learned in season four that, long before they were born (but after this snippet takes place), she made a choice that would ultimately play a role in helping to bring about the Apocalypse.
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Local legend held, in a quiet, low-key sort of way, that a part of the Seine was haunted by a policeman who'd drowned there well over a century ago.
The policeman's legend amongst hunters was much greater, known worldwide, because this particular spirit wasn't a typical ghost at all. Spirits who lasted that long became scattered, lost who they were, turned violent and angry and forgot their own names. This ghost, on the other hand, was calm, coherent, violent only when preventing a would-be crime in progress, and gave his name as Inspector Javert.
It was also worth noting that his death had been the last to occur at that part of the Seine.
In 1969, when the Campbells and their teenage daughter took a vacation in Paris, there hadn't been a reliable sighting of Javert in years. That didn't worry Mary much - a reliable sighting, in this instance, was a sighting by a hunter, and they didn't seek him out often. Why would they? He didn't cause any harm, and French hunters turned to more efficient sources of information first. (And, of course, no hunter would be caught dead sightseeing at a haunted area.) But Mary didn't care about any of that. She was, at that age, already tired of the endless same patterns of the hunter's life, and she wanted to see something different. So, first chance she got, she snuck out of the hotel room and went down to the river.
She had only been there a few minutes when the temperature made a sharp drop; Mary shivered, pulling her jacket tighter around herself, and turned.
"It's late, Mademoiselle," Javert said. Mary knew some French, from classes, but it didn't seem to matter at that moment; the words bypassed her ears, untangling in her brain, and later she would never be able to figure out if she had heard English or understood French.
"Yes, Inspector." Entrenched hunter's training sent Mary's hand into her pocket, where she always had a vial of salt. She knew she wouldn't need it, but she wrapped her fingers around it anyway without even thinking about it.
Javert glanced toward her pocket, then turned and looked down the street the way she had come.
"Someone has followed you."
"I know how not to be followed," Mary said, but her heart rate spiked and she straightened up, scanning the area.
"There," Javert said softly, gesturing with his chin and drawing his cudgel out from the folds of his jacket. Mary let go of the salt and slipped her hand under her jacket instead, drawing a knife. Javert raised an eyebrow at the blade; Mary raised hers right back, challenging, and Javert looked back toward the shadow with a faint smile.
For a long, tense moment, nothing happened at all. Both of them, almost simultaneously, adjusted their grips on their weapons in readiness.
Then the shadow made an irritated sound.
Mary relaxed with a long sigh. "Dad?"
Samuel Campbell stepped out of the shadow, hands held open and empty in front of him.
"It's all right, Inspector." With the initial flash of relief past, Mary could feel her face getting hot. She should have known that she'd gotten past her parents and out of the hotel way too easily. She should have expected her father to be following. "He's my father." She put her knife away, and Javert followed suit with his cudgel.
"You're not good enough to sneak past me yet, Mary." His tone was neutral in deference to the fact that they weren't alone, but Mary could still hear how much trouble she was in. "I apologize for my daughter's disturbing you, Inspector. She's got the knowhow, but she hasn't picked up the common sense about when to use it yet."
"She has to learn somehow." Javert turned to Mary and studied her, cool and assessing. "Be mindful, Mary," he said after a moment, looking into her eyes with a stare that pinned her to the spot. "Think very carefully about the choices you make."
Then he was gone.