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- "All due respect, you're not going to solve this with boys in bullet vests. You gotta let us bring him in."
"And how will that end any differently from the last time."
"Because this time I won't be wearing Loafers and a silk suit. 72 hours, guaranteed."
"36 hours. Barnes. Rogers. Wilson."
"Thank you, sir."
- ―Tony Stark and Thaddeus Ross
Bucky Barnes had been framed for a bombing attack on the Vienna International Centre the day the Sokovia Accords were being signed. Barnes' friend Steve Rogers, otherwise known as Captain America, brought it on himself to bring Barnes in, knowing that ordinary humans would not survive an encounter with a super soldier, especially one that had once nearly beat him to death. However, Barnes tells Rogers that he does not do assassinations anymore and that the attack in Vienna was not his. A chase through Bucharest only resulted in Barnes, Captain America, Falcon/Sam Wilson and Black Panther/T'Challa being arrested. All four were escorted to a Joint Counter Terrorist Centre Building; the two Avengers and Black Panther were placed in an office and had their equipment confiscated, while Barnes was heavily restrained in a mobile cell, to be psychologically evaluated.
Prior to Barnes' interrogation, Theo Broussard, the psychiatrist who was supposed to be carrying out the evaluation, was murdered by Helmut Zemo, the true perpetrator of the Vienna bombing. Zemo impersonated Broussard, and carried out the evaluation as normal until he was able to speak a trigger phrase intended to activate Barnes' Winter Soldier programming and send Barnes on a rampage; Captain America realized that "Broussard" was an impostor, but was too late to prevent the attack. After killing several Joint Counter Terrorist Centre employees and nearly killing Black Widow, Barnes attempted to escape in a helicopter, but Captain America prevented him from leaving, causing Barnes to crash it. Captain America saved Barnes from drowning after falling into the river, and Barnes' Winter Soldier programming had worn off by the time he awoke.
The Joint Counter Terrorist Centre gave Iron Man 36 hours to capture Barnes and bring in his renegade comrades. Although War Machine/James Rhodes, Vision, Black Widow/Natasha Romanoff and Black Panther agreed to help Stark, he still felt undermanned and recruited high school student Peter Parker, who Stark had managed to discover was a superhero named Spider-Man, under the pretense of giving him a educational grant under the Stark September Foundation program.
Before disappearing from the airport, Natasha Romanoff tells Stark that Barnes was framed by Helmut Zemo, and Stark later finds evidence confirming this. Stark follows Rogers and Barnes to the HYDRA Siberian Facility, and is shadowed by T'Challa. They discover that the other Winter Soldiers have been killed by Zemo, who is Sokovian and wants to punish the Avengers for his family's death. He shows them footage revealing the Winter Soldier killing Stark's parents in 1991.
Enraged to learn that Rogers was aware of this (as Arnim Zola had revealed this to both Steve Rogers and Natasha Romanoff two years prior) Stark turns on Barnes. Stark knocks away Rogers and grabs Barnes as Zemo retreats. Rogers throws his shield, deflecting an otherwise fatal repulsor blast. Iron Man restrains Rogers as he once again goes after Barnes, who manages to disable the repulsor gauntlet with his Prosthetic Arm. Iron Man launches a rocket which destroys one side of the room as Captain America destroys the restraints holding him back. As Stark is pinned down from the rubble. Barns takes his chance and runs to an escape hatch located at the top of the facility. Rogers attempts to talk Stark down but Iron Man attacks Barnes again, forcing Rogers to brake one of Iron Man's rocket boots.
In the resulting fight, Stark blasts off Barnes' Prosthetic Arm as he was attempting to remove Stark's Arc Reactor. Rogers manages to use his shield to destroy Stark's Arc Reactor, effectively shutting down his suit.Rogers departs with Barnes and leaves his shield, built by Stark's father, behind. Satisfied that he has irreparably fractured the Avengers, Zemo attempts suicide, but is apprehended by T'Challa.
In the aftermath, T'Challa hands Zemo over to Everett Ross, where Zemo is imprisoned. Stark provides Rhodes with exoskeletal leg braces to let him walk again. Rogers breaks his comrades - Scarlet Witch/Wanda Maximoff , Falcon, Hawkeye/Clint Barton and Ant-Man/Scott Lang - out of detainment and flees to Wakanda, where T'Challa grants them asylum and Barnes chooses to be frozen until a cure for his brainwashing can be found.