Far in the future, the lifeforms known as Gadoll suddenly arose as a threat to humanity. The last surviving humans on Earth confine themselves to the Tank, a lower district in the giant mobile fortress Deca-Dence. While the Gears who live on the upper floors are warriors who go out to fight as part of the Power, most Tankers are content to provide support from the backlines, butchering Gadoll meat and reinforcing defenses. Natsume is among those who would rather go to the front lines; undeterred by her prosthetic right arm, she seeks to join the small number of Tanker soldiers who join the Gears in combat. But despite her peers at the orphanage each receiving their work assignments, Natsume’s enlistment to the Power remains unapproved. In the meantime, she begins a job as a cleaner in an armor repair team led by the hard-nosed and apathetic Kaburagi, who seems to be more than he lets on. Though initially cold to his idealistic subordinate, he soon recognizes in her the potential to upset the status quo of the world. As Natsume’s new mentor, Kaburagi prepares her for the special and unique role as a game-changing bug in the system. Many years have passed since humanity was driven to the brink of extinction by the sudden emergence of the unknown life forms Gadoll. Those humans that survived now dwell in a 3000m-high mobile fortress Deca-dence built to protect themselves from the Gadoll threat. Denizens of Deca-dence fall into two categories: Gears, warriors who fight the Gadoll daily, and Tankers, those without the skills to fight. One day, Natsume, a Tanker girl who dreams of becoming a Gear meets surly Kaburagi, an armor repairman of Deca-dence. This chance meeting between the seemingly two opposites, the girl with a positive attitude who never gives up on her dreams and the realist who has given up on his, will eventually shake the future course of this world.