"History is a continuing cycle of Revolution... War... Peace... Like the three steps to an Endless Waltz....."
Endless Waltz is a three-episode OAV that was released  after the television series. It is described as both a sequel and a presequel to Gundam Wing.  But if you have seen Endless Waltz it's pretty clear that it is set after the t.v series  (it takes place in AC 196, while the TV series was in AC 195).

Endless Waltz is the sequel for  Gundam Wing.  It is set after the events on AC 195, where Earth has finally achieved peace, thanks to the 5 Gundam  pilots.  In a time of peace, Gundams were no longer needed, and Quatre proposed the idea of the destruction of the Gundams.  All the pilots agree, except for Wu-Fei, who has disappeared after the war against White Fang..  When the Gundams were collected and sent to a one-way trip to the sun, a crisis breaks out! Relena is kidnapped, while a powerful military group known as the "
Barton Foundation" begins to takeover various space colonies.  Without the Gundams, the main characters face an extremely difficult situation.  There's frequent "flash-back scenes" through out Endless Waltz, and they  give you a better understanding of the origin and history behind the original series.  There's also more background info on the Gundam Pilots, as well as their training and how and why they were selected.
Endless Waltz
Endless Waltz is divided into 3 episodes:
Episode 1:
The Silent Orbit
Episode 2:
Operation Meteor
Episode 3:
Return to Forever
These three were later combined into a single movie in 1998, titled, Gundam Wing Endless Waltz: Special Edition (
which was shown on Toonami).
The whole Endless Waltz Summary
One of the most noticable changes in Endless Waltz are the new looks of the Gundams.  Each Gundam has been given a complete make-over, with more fancy accessories and outrageous, daring designs.   I have to say they look very cool, ecspecially Wing Zero and Gundam Heavyarms.  The design for that gundam was fantastic.  They made it have the personality of Trowa (by adding half a mask).  As for Wing Zero, the famous "white angel wings" simply took my breath away.  But check it out for yourself!
Want to know more about the Endless Waltz series.  Go to Endless Waltz by Trunks It has a lot of info on the whole story.
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