The story tells about the sudden appearance of unknown life form "Gadoll", which has put mankind in danger of extinction for many years. In order to protect themselves, the surviving people built a huge mobile fortress "DECA-DENCE" at a height of 3,000 meters. The residents of DECA-DENCE are divided into warriors "Gear" who fight Gadoll day and night, and "Tanker" who have no fighting ability. Natsume, a girl who longs for and dreams of becoming a warrior, and Kaburagi, a cold armor repairman, are two seemingly opposite girls who strive to move forward without giving up hope and a realistic man who has given up his dreams. Their encounter has greatly changed the future of the world.