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 a faction below the Sea, 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. As monsters from the depths of the Sea begin to surface near the Mainland's shores, Flitch must also navigate a crisis closer to home. As Flitch, his family, and their allies search for solutions, the truth they seek may lay hidden in old stories and long-held family secrets.

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