Humanity was driven to the brink of extinction by unknown life forms Gadoll. Until one day, Natsume, a girl who dreams of becoming a warriors meets Kaburagi, an armor repairman. They will shake the future of this world.

In the distant future, the human population has been decimated fighting the mysterious creatures known as Gadoll. The few survivors band together to build a mobile fortress known as Deca-Dence, from which they can mount their resistance to the enemy. Natsume has wanted to be a soldier since she was a child, but she is instead sent to work under the apathetic armorer Kaburagi. One day, however, they become embroiled in a battle with the Gadoll while working, and Natsume witnesses Kaburagi take down one Gadoll after another.

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.—brownmisan