On planet Prester, the era of aviation is flourishing with the rise of vintage yet futuristic flying machines known as vanships. Claus Valca, a natural born flyer, and his bold co-pilot Lavie are determined to become the first couriers to cross the Grand Stream in their vanship. However, their ambition is disrupted as they encounter a perplexing young girl named Alvis. Caught in the midst of a ceaseless conflict between rival nations Anatoray and Disith, who are battling it out with chivalric tactics, Claus and Lavie must try to bring harmony to their world by deciphering the secrets of its chaotic center, all while experiencing the consequential loss of lives and the formation of legacies.