Moon, once a solitary wanderer, has become consort to Jade, sister queen of the Indigo Cloud court. Together, they travel with their people on a pair of flying ships in hopes of finding a new home for their colony. Moon finally feels like he's found a tribe where he belongs.

But when the travelers reach the ancestral home of Indigo Cloud, shrouded within the trunk of a mountain-sized tree, they discover a blight infecting its core. Nearby they find the remains of the invaders who may be responsible, as well as evidence of a devastating theft. This discovery sends Moon and the hunters of Indigo Cloud on a quest for the heartstone of the tree — a quest that will lead them far away, across the Serpent Sea.