Bruce Wayne never went to the circus on the day the Graysons died, meaning he never adopted their orphaned son, Richard.
Dick went to a Juvenile Detention Centre for eight months before spending a year going in and out different foster homes, each one rejecting him, now he's in a Gotham orphanage.
Five years after the Graysons death, and there's a new black haired, blue eyed thief in Gotham, nobody knows who he is, it's a mystery. So that's what they've called him- Mystery. Mystery constantly annoys the Justice League and their sidekicks, occasionally helping out when it suits him.
The Justice League know nothing about the young boy and its starting to frustrate them, especially Batman.
What happens when they find out that Mystery is just Richard Grayson, a messed up orphan? Will they decide to send him back to Juvie or help him?
Unknown to the Justice League, he's running from something, but what?
Disclaimer- I don't own Young Justice