Percival Bellwater is a scion of wealth and privilege, attending the elite airship academy on Mance. He can pass his classes without trying, so he never tries very hard, in spite of his father's beatings. His classmates all know he can run circles around them with nautical math when he chooses to exert himself on his studies. Percy just prefers to exert himself on cards, rum, and sailor lads.

Two years ago, Percy met a freckled redhead from the lower deck named Silas Ackelby. Silas has immense ambition, a mind like a gear-driven ship, and a rare gift with cards. He's proud and prickly, and he loves airships more than he loves to breathe. Percy is fascinated. They quickly develop a complicated friendship, Silas staying in Percy's guestroom during the off-season, gambling and carousing with the students throughout the winter. He and Percy exchange letters all summer when the airships are working and the students are in classes.

When Silas comes home early one fall, Percy skips studying for his exams in favor of a night on the town with his best friend. The repercussions prove more severe than either of them anticipated, threatening their friendship and even their lives.