Az Gabrielson is one of the Airborn, a people who, with a stretch and a beat of their eight-foot wings, travel effortlessly around and between their cities, perched high above the clouds amidst a life of ease and airy beauty. Az, however, has no wings, making him a pained and isolated oddity in his glorious world of freedom and flight. Then one day he is selected for a job below the clouds. The system of massive automated elevators, which send up everything the Airborn need to survive, are breaking down—and threatening to take the Airborn society with them. Someone must travel to the Ground to find out what has happened, and Az, with his wingless similarity to the prehistoric Groundlings, seems perfect for the task of hunting for answers beneath the clouds. But in the vast shadows of the cities on the Ground, Az finds more questions than answers when he discovers that the Groundlings worship a dim notion of the Airborn and aspire to be like them. Filling the elevators with tributes to their winged deities, the Groundlings are beginning to think that their way of life is part of a very unnatural order of things. Az also meets a girl named Cassie Grubdollar, who's definitely no angel!

The Groundlings are under attack. Approaching in darkness in huge airships, Airborn pirates are raiding refineries and pillaging Groundling factories - taking by force the supplies which used to be offered as a tribute to the winged 'gods'. And the Groundlings are getting ready to defend themselves. Above the clouds, Mr Mordadson is dispatched to Redspire, the heart of the piracy, to solve the problems however necessary and put an end to the piracy ...and taking the armed Airship Cerulean - with Az Gabrielson as Captain - with him. But even on the ship there is unrest. Flight Lieutenant Wallimson hates Az, and plots to do whatever he can to undermine the wingless boy...and on the ground, unrest at Redspire is the last thing on Cassie's mind. The family business has been destroyed by contact with the Airborn, and they scrape a precarious living taking Airborn tourists on tours. But as trouble is stirred up between the Groundlings and Airborn things go from bad to worse for Cassie. And then her father goes missing ...