In U.C. 0123, the aristocratic Crossbone Vanguard seizes the newly built colony Frontier IV as a base for the foundation of Cosmo Babylonia. Young Seabook and his schoolmates desperately try to escape from the enemy, but his friend Cecily is captured by the invaders, and learns that she is the granddaughter of the Crossbone Vanguard's leader. At the nearby Frontier I, Seabook becomes the pilot of a powerful new Gundam in order to help rescue his friend.