In his continuing adventures, Twig, now sixteen years old, joins the crew of his father's sky pirate ship and embarks on a dangerous mission to collect the powerful stromphrax, a substance that purifies water and also prevents the city of Sanctaphrax from floating away.

Apprenticed to the Most High Academe of Sanctaphrax, a floating city of scholars, gossip, and treachery, fourteen-year-old Quint runs increasingly dangerous errands, which eventually bring him to a place of gruesome monsters that threaten his life and those of his friends.