You're the new member of Young Justice. After you get dropped off at their island base, Mount Justice, you're greeted by Bart as you walk in.

"Are you the new member?" He asks brightly, zip-zipping around you in little flashes of speedster speed, the hyper puppy. "I'm Impulse, but you can call me Bart." He grins widely as he skids to a stop in front of you. "Welcome to Young Justice."