Claus Valca is from the world of Prester where he and his longtime friend, Lavie live and work as sky couriers. They pilot a van-ship left to Claus by his father. Together they dream of one day making the journey their father`s were never able to complete - flying acros the notoriously dangerous stretch of space known as the "Grand Stream."

One day, after Claus and Lavie agree to shuttle a mysterious girl named Alvis Hamilton to the air-battleship, Silvana, they become involved in a war that shakes their world - Prester, a world where mighty wind and a sea of clouds spreads far and wide...