Gundam Wing: Endless Waltz is the sequel to Gundam Wing, which is set in an alternate universe to the original Gundam series in the After Colony timeline.
Spoiler warning: Plot or ending details follow.
Treize Khushrenada is dead and Oz has come to an end. Exactly one year after his death and the end of the conflict between the colonies and Earth, a rebellion occurs on the newly completed colony, L3 X-18999. Led by eight year old Mariemaia Khushrenada, Treize's daughter, kidnaps Relena Darlian, now the Vice Foreign Minister of the Earth Sphere Unified Nation (ESUN), during a diplomatic mission to X-18999. As the Gundam pilots investigate further, they discover that Mariemaia is merely a puppet, controlled by her grandfather, Dekim Barton, a former advisor to martyred colony leader, Heero Yuy. The Gundam pilots must prevent Dekim from seizing power over the ESUN.
Endless Waltz is both sequel and prequel to the Gundam Wing series. It is a sequel as it is a continuation of Gundam Wing, but is also a prequel as it reveals details regarding the pasts of the Gundam pilots and the true objective behind Operation Meteor.
As with much of Gundam Wing, Endless Waltz is a thinly veiled retelling of the Universal Century - in this case, a retelling of Mobile Suit Gundam ZZ, with Mariemaia taking the place of Minerva Zabi, and Dekim in the role of Haman Karn.
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EndlessWaltz was first produced as a three part OVA and later in 1998 a compilation movie was released which also contained new scenes.
As the Gundam pilots investigate further, they discover that Mariemaia is merely a puppet controlled by her grandfather Dekim Barton, a former advisor to martyred colony leader Heero Yuy, who is using X-18999 to go through with the original Operation: Meteor, as a contingency plan in case the ESUN doesn't comply.
When EndlessWaltz was released in theaters in 1998 as a lead-in to Gundam's 20th anniversary, several new scenes were added, adding approximately ten minutes of new footage to the story.