A young writer partners with a literary agency and wealthy backers to buy and outfit a yacht for an expedition to Hyalos, an island near reefs that hides ruins beneath the sea. The narrative traces preparation of deep-sea gear, the assembling of an unusual crew, the voyage and underwater exploration, and the revelation of Martia's secret. Encounters include salvage attempts, legal and commercial tensions with art dealers and authorities, personal dangers aboard ship and on the island, and the moral and practical consequences that follow the quest for a submerged city.