Protecting the batbrothers
Every villain needs money, right? But in that kind of buissnes, it's hard to get it the legal way, so why not kidnapping a rich child for ransom- or even better, four rich children?
That was exactly what the Joker was planning to do, maybe he would even get to kill the bat and his little bird if they'd come to the boys' rescue. But what happens, when the Young Justice gets the mission, to protect Dick Grayson, Jason Todd, Tim Drake and Damian Wayne, four of the most richest children of Gotham? And the team is not at full strength, since Robin has left to some mission with Batman in Alaska, but they can stop the Joker anyway, right? Right?!