In a distant future, humanity faces a grave threat from the Gadoll lifeforms. The remnants of society dwell within the massive mobile fortress Deca-Dence, divided between the fighting Gears above and the support Tankers below. Natsume, determined to defy her limits despite her prosthetic arm, dreams of joining the frontline fighters. Assigned to a cleaning duty under the mysterious Kaburagi, she discovers a hidden world within their confined existence. As Kaburagi reluctantly takes her under his wing, he sees her potential to challenge the established order. Together, they embark on a journey that could reshape their world, with Natsume poised to be a catalyst for change.