Overmorrow

According to Ellie, the middle child of two overseas ambassadors, a magical monster named Oblivion kidnapped her elder brother. She believes Oblivion wants to use her brother as a backup memory, a backup hard drive.

She encounters Overmorrow - a magical rain that wakes up normal New Yorkers unto the wonder all around them.

Once she receives Overmorrow and its "seer's sight," she discovers someone has stolen the source of the rain.

Convinced Oblivion has also stolen the rains, she decides to use the mystery of Overmorrow to lead her back to her elder brother and save his mind from Oblivion. As Ellie fails over and over again, it becomes terribly apparent to her that this rain thief wants to obliviate monsters from the memories of New Yorkers and everyone else in the megacosm.

And if we forget the monsters, they're free to eat us all.