When Universes Collide

The walls between universes were growing weaker and weaker. Looking back, what occurred was inevitable - especially when you have a mad alien in a blue box desperately trying to save the women he loves. It started before then though. The Doctor crash landed in a parallel universe, opening small cracks for a split second.
That was enough.
It was enough to let demons through, demons that Sam, Dean and Castiel had forgotten about, too busy fighting Leviathans.
Our Storybrooke heroes were too busy mourning the loss of Robin Hood and celebrating the return of Killian Jones to notice something was wrong.
Sherlock was enthralled in his game with Moriarty, unaware that he was about to thrown into a situation much stranger than any drug could produce.
As for the Avengers, well, they hadn't even assembled yet.

When the walls between fiction and reality become a little less defined, our heroes must band together to return things the way they were.