The main character of the series, Houichi's life is turned upside down when the Riofaldians take over the Earth by force. Houichi is called "Gun" by his friends, which is a joke on his name. He is called this due to the fact that the kanji symbol for Ho in his name means "gun" in Japanese. On the day of the tenth anniversary of first contact with the Riofaldians, Houichi meets Isaka, a beautiful, not to mention well-stacked, young high school girl who claims to be dating Houichi. She turns out to be a creation of Housuke Kano, and gives Houichi the Gunner Suit Glove, which allows Houichi to change his clothes into a power suit. After fighting off Riofaldian robots, Houichi is taken to the XXX Unit, Exaxxion.