Long ago, the mysterious Gadoll suddenly arose as a threat to humanity. This led to the loss of over 90% of the world population. In this dire situation, the Gears, soldiers belonging to the anti-Gadoll corps called The Power, pooled their great knowledge to build the mobile fortress Deca-Dence. The fortress utilized oxyone, the energy that flows through a Gadoll's veins, to become humanity's last bastion. Within Deca-Dence there exists a caste system of the Gadoll fighters, Gears, and the non-combatant people who support them, the Tankers. Natsume, a girl who is an orphan with an artificial arm as a result of a Gadoll attack, hopes to become a Gear but is looked down upon.