A Robin's Nest

When he was a kid, Midoriya Izuku was hit by an unknown Quirk. He was found a few days later, unharmed. He was gone for just a few days to the world, and he never told anyone where he'd been. Who would believe that he'd been sent back in time several hundred years to the Pre-Quirk Era? That he'd been adopted by a pair of circus performers who gave him the name Richard Grayson? That he, Quirkless Midoriya Izuku, had become Robin, the Boy Wonder, famed partner to the Old Hero, Batman.

Yeah. Nobody was going to believe that. At least he could become a LEGAL hero, now.