Ciel is a world of floating islands adrift in an infinite sky. A small number of people born on Ciel manifest supernatural abilities known as Gifts, and the rarest Gift of all is the ability to bond a wild Leviathan. These floating beasts grow to the size of galleons and spend their entire lives in flight. They come in forms ranging from eight-flippered tortoises with opalescent shells to marble stallions tall as grain silos, six-legged with eyes like burning coal and dancing manes of fire. The Leviathan and their Gifted pilots rule the Eight Skies of Ciel, transporting people, moving cargo, leading armadas... and engaging in a few more unsavory trades.

Effie Strait dreams of Leviathan. Born on the Island of Volturnus in the remote Zephyr Archipelago, Effie knows little of the world beyond her local isles, but that fact hasn't throttled her ambition. When she was young, her homeland came under assault by the notorious pirate, Maug the Butcher. Effie's people were saved from enslavement by the timely intervention of the Celestial Armada, but that salvation came at a price. The Celestials remained in the archipelago, imposing a tithe in exchange for their ongoing protection and importing a strict caste system that elevates the Gifted above their mundane peers.

Once a year, the Celestials come down from their palace on Aquilon Island to test the most promising young Zephyri against a pod of wild Leviathan. Those identified as potential pilots are whisked away to the Celestial homeland far across the open sky, never to be heard from again. Effie yearns to prove herself in the testing, but not every Zephyri shares her disposition. When a rebellion against the Celestials erupts on the neighboring Isle of Aeolus, the fighting draws in Zephyri from around the archipelago--including Effie's sister Vanna and her best friend Kai. With Effie's own time of testing approaching, she finds herself caught between her closest friends and family and the only power on Ciel that can make her piloting dreams a reality.