The world of Prestel, where the polluted atmosphere of the industrial revolution still lingers; where, underneath the wind and clouds, two countries, Anatorel and Dysis, confront one another in war; and where a terrible and exciting story is about to begin. There lives Claus Valca, a 15 year old who, using the vanship (a small flying ship) left behind by his father, made his home in the sky along with his childhood friend Lavie Head, who is also 15. Their dream is to surpass their fathers, to do what they couldn't and fly through the eternal storm known as the "Grand Stream". However, one day, a mysterious girl, Alvis Hamilton, aged 11, is entrusted to them by a wounded vanship pilot. With his dying breath, the pilot passes on his mission to Claus, to deliver Alvis as cargo to Alex Rou, captain of the flying battleship Silvana. One thing leads to the next, and soon Claus and Lavie find themselves being dragged into a battle that will decide the fate of the world.