After the transformative journey they took together to find the Monster, Sola and her friends have gone down separate paths. Where Sola has embraced the unknowable, ever-shifting nature of life on the open seas, Hunder finds comfort close to home as the chief of the unified Sun and Fortress Islands.

Then their world is thrown into chaos when Alef, the captain of one of Hunder's ships, brings news of a massive island-a continent-that he says the Sun and Fortress islanders are destined to claim for their own. Alef and Hunder launch an expedition to the continent that threatens to bring death and destruction to any and all who stand in their way. Sola must make a choice-step aside and allow other islanders to die, or risk her own life in order to stop her friend.