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 dreams of becoming a warrior, and Kaburagi, a cold armor repairman, are two seemingly opposite people. The girl who never gives up hope and strives to move forward, and the realistic man who gave up his dream, their encounter has greatly changed the future of the world.