It is A.C. 196—one year after the conflict between Earth and its nearby space colonies ended with the formation of the Earth Sphere Unified Nation. As mankind enters a new era of unity and prosperity, four of the five Gundam pilots decide to follow in society's footsteps and disarm by launching their mobile suits into the sun. However, their plan is unexpectedly suspended when a rebellion erupts with the emergence of Mariemaia Khushrenada—who claims to be the child of fallen dictator Treize and desires to conquer the world in his stead.

Deciding that they are the last line of defense for the Earth Sphere, Heero Yuy, Trowa Barton, Duo Maxwell, and Quatre Raberba Winner scramble to retrieve their Gundams and fight to protect the peace the people of the colonies have pursued for so long. Despite their unwavering resolve to complete one final mission, the battle will be challenging: the pilots' former ally Wufei Chang has pledged his allegiance to Mariemaia's cause instead.