In the year 2075, mankind has reached a point where journeying between Earth, the moon and the space stations is part of daily life. However, the progression of technology in space has also resulted in the problem of the space debris, which can cause excessive and even catastrophic damage to spacecrafts and equipment.

"Never lose that strength or nobility, even when you grow up." When Utena was just a child and in the depths of sorrow, she found salvation in those words. They were the words of a prince, who bestowed upon her both a ring and the promise that it would lead her to him again. She never forgot the encounter. Now a teenager, Utena attends the prestigious Ohtori Academy; however, her strong sense of chivalry soon places her at odds with the student council and thrusts her into a series of mysterious and dangerous duels against its members.

100 years ago, the last remnants of humanity were forced to retreat behind the towering walls of a fortified city to escape the massive, man-eating Titans that roamed the land outside their fortress. Only the members of the Scouting Legion dared to stray beyond the safety of the walls – but even those brave warriors seldom returned alive. Those within the city clung to the illusion of a peaceful existence until the day that dream was shattered, and their slim chance at survival was reduced to one horrifying choice: kill – or be devoured!

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.