Moanna Morgan had never seen the sky. No-one from the Tethys Colonies had, not for a thousand years. Not since the Old Earth died and the world of Thalassa was born, a new world beneath the waves… Submarines sank all the time out in aqua incognita, beyond the Federation Frontier, and Moanna’s brother had been dead for nearly a year. But was the wreck that was found really his? And if not, where did he go, and who would fake it? A chance discovery sends Moanna on a journey into the unknown. There she comes face to face with a secret that has remained hidden through all the ages of Thalassa and unleashes a threat that could destroy everything she knows…

Moanna Morgan went to the ends of the Earth to find her brother – she found the end of the world. The Tethys Federation is under attack. For ten years there has been peace. For ten years, the Tethyans believed that they alone ruled under the waves. They were wrong. Beyond the Frontier, hostile forces are preparing for war. Pioneers and Colonists are afraid. Tensions are running high. Old divisions have never healed. Scattered throughout the Colonies, the seeds of destruction have already been sown. The end is only a matter of time. The Sunken City will stop at nothing to protect its secret. And hiding out in Aqua Incognita is the one person who can save them all…

The Sunken City has been destroyed, betrayed by Jason Morgan, but the Federation will not celebrate its victory for long. The threat of the Omega toxin is looming, and Moanna Morgan’s attempt to steal the vaccines has failed. Now she is in custody and facing the scant mercy of Naval Intelligence. Beyond the Frontier, the unhinged and vainglorious Garrett McCall marshals his forces, ready to strike a deadly blow for the Pioneer cause. Alone and adrift in a floating tower, ex-MANTArine McPhail needs a way out, and fast. And Hamilton Khan is ready to set off in search of the buried gold reserves in Old New York, but an old enemy has other ideas. All across Thalassa, events are moving towards a final confrontation. At the heart of it lies a conspiracy many years in the planning, one that will sweep everything else away...