Endless Waltz is the OAV/Movie which takes place a year after the series ends. It's told in three episodes, and there are several versions available. Cut and edited, which leave out not only the blood and cursing, but cut the movie short by a few scenes. Unedited but cut, which includes the blood and cursing, but leaves out a few scenes. And finally, Uncut and unedited, which include the cursing, the blood, Dorothy, and a conversation between Duo, Heero and Quatre. The final one is the Special Edition. Spoilers Ahead.
In AC 196, one year after the series ended, Quatre decides it's time to destroy the Gundams. After all, during a time of peace, there is no use for weapons of mass destruction. Heero, Duo and Trowa all readily agree to this. Wufei of course, never answers. Quatre and Duo send the four Gundams on a resource block towards the sun.
Naturally, the moment the Gundam pilots don't have their Gundams, a new threat makes its presence known. The Barton Foundation, the original schemers behind the original Operation Meteor, are hoping to relaunch the plan in order to take control of the Earth's Sphere. Dekim Barton is in charge of this operation, and he plans to use Treize Kushrenada's daughter, Mariemaia, to achieve his goals. Mariemaia has been brainwashed by her grandfather into believing she wants to be victorious and rule the Earth. She kidnaps Vice Minister Relena Dorlian/Peacecraft in order to fulfill her grandfather's objects. Wufei has decided to help Mariemaia achieve her goals by lending her the power of his Gundam. Trowa tries to infilterate the Barton Foundation, but is captured by Wufei.
While Preventers Sally (codename Water) and Noin (codename Fire) try to figure out where the the terrorist group is located so they can stop them, Quatre takes the Maguanac Corps with him to retrieve the Gundams, which are somewhere around Venus at this point. Lady Une, in charge of the Preventers, wonders how she's going to ever stop Mariemaia, when her office door opens and a rather unexpected guest enters, asking for the Preventer codename of "Wind" so he can assist in stopping Dekim Barton.
While Duo and Heero break aboard one of the Barton Foundation's bases in space, Trowa helps them out and joins them. Preventer Wind attacks Dekim as Dekim and Mariemaia head to Earth, destroying most of Dekim's suits before he's put into a checkmate and forced to stop fighting.
Mariemaia and Dekim make it to Earth and seal themselves in the Brussel's Presidental Mansion, which drops some 7 floors down into the ground. Zechs (codename Wind) and Noin take their mobile suits to try and take out Dekim's army alone. Quatre gets the resource block back just in the nick of time. He meets Duo, Heero and Trowa. While Heero goes to battle Wufei, the other three head to Earth to help Zechs and Noin.
It takes Heero a while, but he finally gets through to Wufei. Duo, Quatre, Trowa, Zechs and Noin run out of firepower and debate on self-detonating to take out the remaining troops when Heero shows up to destroy the underground mansion. Wufei sides with them by rallying the people of the town. Heero fires three shots, destroying the mansion, and Wing ZERO crashes into the ground and Heero falls out. Lady Une hastily gets into the control room of the mansion to confront Dekim and Mariemaia. She saves Mariemaia once, but Mariemaia is shot by her own grandfather. One of the men under Dekim proves to be a traitor and shoots Dekim. Heero appears and pretends to "shoot" Mariemaia before collasping onto Relena. As Lady Une rushes Mariemaia to the hospital, Relena holds Heero close and whispers that the war is finally over.
In the end, Duo, Quatre and Trowa self-detonate their Gundams together, while Wufei detonates his in the middle of China. Zechs and Noin sneak off into space together for some time alone, and Sally tracks Wufei down to see if he'd like to become a Preventer and her partner. Lady Une vows to care for Mariemaia and takes the girl to see Treize's grave. And Heero stands in the background while Relena continues her work as a promoter of peace.