Disclaimer: None of these characters are mine, more's the pity.
Set: Sometime S2 I guess.
Sam Winchester curled his 6.4" frame behind a row of dustbins and tried very hard not to be seen. He could hear the sounds of a fight from the other end of the alleyway and his plan was short and sweet. Get to the other end of the alley without being seen. Dive in, save the girl. He hadn't a clue where Dean was.... he amended that as his brother joined him behind the bins.
"You OK?" Dean whispered.
Sam nodded and then winced as the sound of someone getting thrown into a pile of dustbins echoed down the alleyway.
"So we're going to go help her then?" Dean asked, poking his head up from their hiding place.
Dean flashed Sam a grin. "My brother, always a gentleman."
The Winchesters sneaked from the dustbins made their way swiftly and silently to a screaming halt in a doorway as they saw the figures fighting.
"Dude....." said Dean after a moment. "Is that......"
"Jo?" Sam said softly. "Looks like."
They watched in silent awe as the petite blonde cracked the vampire she was fighting across the face sending it backwards to sprawl on the ground. She brought out a stake from her jacket pocket and leaned in and stabbed downwards. Nothing happened and she frowned.
"The heart Jo," a man's voice came from the behind the figures. "The heart is further over."
She hit the vampire again to ensure it stayed down and then stabbed downwards again with her wooden stake, the vampire exploding into dust beneath her. She rose to her feet, brushing the dust from her clothes.
"Well done." The man came into Dean and Sam's sight. He was tall and broad shoulders, dark hair that felt across his forehead and, as he turned in the streetlights, an eye patch.
"Not really," she said. "I always seem to miss first time."
"Slayers can't afford to miss. But nowadays they can afford to practice." The man patted Jo on the shoulder. "You'll get there. Now then," his voice changed a little. "Who are our friends hiding in the doorway?"
Jo flicked a glance over to the Winchesters who stepped out of their hiding place somewhat sheepishly.
"Hey Jo," Dean said amiably. "Looks like you've been having some fun of your own."
"Dean and Sam Winchester, Xander Harris." Jo introduced them briefly. "They're John Winchester's sons."
Xander held his hand out to them and shook their hands firmly. "Pleased to meet you."
"Not hunting alone now Jo?" Sam asked. "Everything OK?"
"Everything's fine Sam." Jo looked up at him, the smile on her face lighting up her eyes. "Turns out you're not the only ones with a calling. I was called as a Slayer. Xander's my Watcher."
"Wow," Dean breathed clearly impressed then, "What the hell is a Slayer?"