Relena, now a deputy Foreign Minister for the ESUN, is abducted while on her way to colony X-18999. To her astonishment, the one responsible for her abduction is Mariemaia Khushrenada, Treize's daughter and the granddaughter of Dekim Barton, leader of the Barton foundation. However, it soon becomes clear that Mariemaia is being manipulated by her grandfather to complete his second scheme of total domination of that Earth sphere. Heero and Duo, having learned of Relena's abduction, decide to investigate and send Quatre on a dangerous mission to recuperate the Gundams, while Trowa learns about Dekim's latest bid for power and decides to stop him. However, as he tries to kill Dekim, Trowa is stopped by Wufei, now a member of Mariemaia's forces!

As they arrive at X-18999, Heero and Duo are met by Wufei and Trowa, who fire at them. It soon becomes evident that they were faking things only to make it easier for his friends to infiltrate the colony. Meanwhile, Mariemaia declares war on the ESUN and departs for Earth with Relena. At the same time, a lone MS, piloted by Zechs Merquise, now a ESUN agent, approaches the asteroid MO-III, where Mariemaia's forces prepare themselves for their impending assault on Earth and threatens to blow it to pieces. Dekim's only answer is that if Zechs so much as sneezes, he will drop the asteroid on Earth.

Mariemaia's forces descend on Brussels, the ESUN's capital, where Mariemaia, her grandfather and Relena go into a huge, super-armored bunker. Meawhile, Zechs and Lucrezia Noin try, without success, to stop Mariemaia's MS troops, until the Gundam Heavyarms, Sandrock and Deathscythe H arrives, that is, with the only two missing Gundams duking it out during atmospheric re-entry.