When sixth-grader Rosie begins to see magic, she learns that her mother's dwindling memory is tied to an age-old battle between the light of the moon and the darkness of witches.

Accompanied by her loyal friend Germ, twelve-year-old Rosie Oaks travels through time and space in a magical whale as she sets out to save her brother and fulfill her destiny to find and vanquish the remaining Thirteen Witches.

Thirteen-year-old Rosie and the League of Witch Hunters, joined by a gaggle of the world's last ghosts, cross the galaxy in search of the Museum of Imagined Things, the last treasure the Nothing King wants before dragging everything into oblivion.