In the future, something dangerous threatens mankind. People have to hide in a mobile fortress called Deca-Dence. Up above are the Gears, who fight for the Power against the danger. Most people in the Tank, the lower part of the fortress, just help from the backlines. But Natsume, who has a prosthetic arm, wants to fight too. She tries to join the Gears, but she can't. So she starts a job as a cleaner and meets Kaburagi. He sees potential in Natsume and helps her prepare for a special role in the fight against the danger.
Natsume will have to use her courage and skill to face the threat and save humanity. Will she be able to do it?