Nestled within the Mud Whale, a vessel that navigates ceaselessly through a sea of sand, exists a community segregated into two distinct groups: the Marked who possess the ability to manipulate "thymia" at the cost of their life span, and the Unmarked who cannot, but hold authoritative positions and lead normal lives. Suffering from hypergraphia, the archivist of the Mud Whale, 14-year-old Chakuro, bears the responsibility of documenting every event that occurs on the island and unexplored territories. While on a quest to discover an uncharted island, Chakuro stumbles upon an injured girl with no recollection of her past. Attacking him upon their encounter, she loses consciousness and is brought to the Mud Whale by Chakuro. As no one has ever encountered an outsider before, the mystery surrounding the girl, nicknamed "Lykos," thickens further with her cold demeanor and exceptional combat skills, hinting that her life experiences deviate greatly from those within the Mud Whale. With the arrival of Lykos, Chakuro is hurled on a fateful journey beyond the shores of the Mud Whale, forever altering his destiny in ways he could never fathom.