After barely escaping Earth with the League of Witch Hunters, Rosie and her friends are hiding out from the Nothing King and his witch followers on a barely inhabited planet. Then a messenger arrives with the unexpected news that Earth survived the Nothing King's black hole, but only because he wants one last treasure before dragging everything into oblivion: the Museum of Imagined Things. Rosie saw the museum once when she visited the Brightweaver in the clouds. It's infinitely tall and made only of mist and figments, so Brightweaver was able to bundle it up and hide it for safekeeping.

The League of Witch Hunters, joined by a gaggle of the world's last ghosts picked up from Limbo along the way, cross the galaxy in search of the museum and in preparation for their last showdown with the Nothing King. As Rosie and her allies weather surprises and betrayals while fighting to maintain their trust in each other, they may find the museum has one last secret in store.