The story takes place in Japan. The year is 2030 and Earth has been completely overrun by a viral disease named "Gibia" that turns infected people into various different monsters, based on their age, sex, and race. A samurai and a ninja from the early Edo period travel together through time and arrive in a ruined Japan to aid a professor working on a cure for the virus. Together, they fight countless Gibia monsters, outlaws, and other fierce foes on their journey to save mankind.

It has been half a century since humans took to the sky to hunt dragons. Although dragons present a great danger, every part of them is extremely valuable, and brave dragon hunters, called "drakers," risk their lives aboard special dragon-hunting airships to pursue them. However, despite the thrill and excitement of hunting, the life of a draker is not easy. Ostracized by society, the drakers have no place to call home. Life in the air is not any easier either: maintaining an airship is dull and expensive—and dragons are even harder to catch, as each one is unique. One of the last remaining such airships is the Quin Zaza. Its courageous crew includes Takita, an eager newcomer trying to earn her place on the ship; Mika, a reckless but highly skilled hunter; and Jirou, an earnest young man chasing both the dragons and his past. United by the same goal, these hunters face arduous aerial battles that involve daring feats of skill, strategy, and more than their fair share of danger.

Ten years ago, the World Tree that protected peace and order of the world was being targeted by an evil organization called the Zakum. The Human race led four other races: the brave warriors, the intelligent magicians, the agile archers and the sly thieves in a fight against the Zakums. In the end, the World Tree activated self-destruction to protect itself. However, the other races thought it was the Human race that destroyed the World Tree. When the World Tree was destroyed most of the races had nothing left, so a number of them went to Maple Island thinking the ones who stayed would die. They escaped Victoria and lived on Maple Island where living is getting harder and harder. Since then, a never-ending war between the different races began. Ten years later, the main protagonist, Al, is determined to revive the World Tree. Al and his companions begin their journey to seek the seeds of the World Tree. At the same time, the Zakums reappear… (Source: Wikipedia)