Seabook Arnou is a mechanic engineer who lives a pacific life within his home colony of Frontier IV, however all of this will change when the "Space Pirates" known as the Crossbone Vanguard will attack Frontier IV in an attempt to take the colony as their home base.

Much to his surprise, Arnou will learn that he has strong ties to the conflict, not only his mother has heavy involvement with the Gundam Formula 91 project, but his childhood friend Cecily Fairchild seems to be an important target for the Crossbone Vanguard. Seabook will have no choice other than pilot the Gundam designed by his mother, and attempt to put a stop to this growing conflict.