After setting fire to the Forever Sea and leaving the surface world behind, Kindred Greyreach dives below to find a Seafloor populated by roving bands of scavengers. Among them, Kindred discovers a familiar face working to save the Sea from the continued spread of the Greys and the ravages of the world above. But when Kindred finds herself at odds with them, she and her friends will have to use every power available to them—including their link to the surface world—to forestall disaster.

Meanwhile, above, a boy named Flitch, son of the Baron of the Borders, finds himself caught in a dangerous political crisis as survivors from Arcadia and the Once-City arrive on the Mainland. When Flitch begins to receive messages from someone below the Sea, the denizens of the Mainland see it as a sign that ancient enemies from across the Forever Sea are returning. The resulting crisis forces Flitch and his siblings to flee, as they seek out the truth hidden in old stories.

Above and below, Flitch and Kindred will have to work together to save themselves, their loved ones, and the Forever Sea itself.