Imagine a planet entirely covered in clouds, imagine an ocean of cloud, stretching away as far as the eye can see, forever shifting and changing, imagine peaks rising out of the clouds, their gentle slopes covered in fields and orchards, imagine a citadel standing on each peak...Heliopolis, a prosperous citadel divided into five layers, its rigid social structure ensuring that it is run efficiently and the citizens know their place. But there is one uncertainty that all Heliopolitans share. What lies beneath the impenetrable sea of cloud? An abyss of nothingness? The fiery realm of the god, Omnium? 14-year-old Marcus is about to find out. Isolated and lonely, Marcus lives in the soulless palace at the very top of the citadel. When his father fails to return from a diplomatic mission, Marcus mounts an expedition across the cloudscape in search of him. But he discovers too late that danger lies closer to home - somebody wants him dead. And when his flying vessel is sabotaged, Marcus finds himself plunged into a terrifying new world beneath the clouds...