Located inside the Mud Whale that endlessly soars through a sea of sand, there is a community divided in two: the Marked majority who are capable of using a power called "thymia" at the cost of their lifespan; and the Unmarked, who are unable to wield such an ability but have regular lifespans and hold positions of authority. Diagnosed with hypergraphia, 14-year-old Chakuro is the archivist of the Mud Whale, with the responsibility of recording everything that happens on the island and other uncharted places. During one mission to explore a foreign island, Chakuro encounters a wounded girl with no memories. She immediately tries to attack him, only to pass out and be taken to the Mud Whale by Chakuro himself. The girl's existence is shrouded in mystery, as no one has ever met a human from the outside world before. Furthermore, her lack of emotions and skilled combat abilities demonstrate that her life experience is vastly different from those inside the Mud Whale. With the sudden appearance of the girl nicknamed "Lykos," Chakuro is about to uncover what lies beyond the Mud Whale—forever changing his destiny in the most unbelievable way. [Written by MAL Rewrite] Included one-shot: Volume 7: Hikari no Tonari Kabe no Sora (The Sky on the Wall, Next to the Light)