Many years have passed since human beings fell into the danger of extinction, due to the sudden appearance of unknown creatures known as "Gaddle." The survivors built a huge mobile fortress called "Deca-dence",whose total height is about 3000M to protect themselves from the threat of "Gaddle."

People in Deca-dence are divided into two classes: one is "Gears", who fight against "Gaddle" day and night,and the other is "Tankers", who have no fighting power. One day, Natsume, a "Tanker" girl who dreams of becoming a "Gear", a warrior who fights against "Gaddle", meets Kaburagi, a ruthless Deca-dence armor repairman.
An optimistic girl who never gives up, and a realistic man who gave up all of his dreams, their meeting will shake the future of this world.