Fourteen-year-old Chakuro lives aboard the Mud Whale, a giant island-vessel that drifts across an endless sea of sand. Like most of its five hundred inhabitants, he is a Marked: a short-lived psychic who can wield the power called Thymia. He spends his days as the ship's archivist, recording the people around him before his own time runs out. When the Mud Whale comes upon a derelict island in its ninety-third year of exile, Chakuro finds a silent girl named Lykos hiding inside, and her arrival unsettles everything the islanders thought they knew about their home.