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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. Iron Man used his lasers to trap Captain America from him, returning his focus onto Barnes.
Iron Man fought Barnes who was trying to escape to the top of the building, Captain America and destroyed Stark's armor targeting system, leaving the former to have to use his own eyes to destroy the hatch. When trying to throw his shield again, Iron Man shot the shield mid-air, leaving Captain America defenseless and continued his attack on Barnes.
Having Barnes trapped inside, Iron Man fought him in hand-to-hand combat until he had him in his grasp. Iron Man questioned did he remember his parents, with the latter stating he remembered everyone he killed for HYDRA. Captain America jumped on the pair and knocking himself and Stark to the ground below. Captain America insisted that killing Barnes wouldn't change anything, but Iron Man stated he didn't care, wanting to avenge his mother's death, and was willing to kill Rogers to get to the Winter Soldier. The two battled in hand-to-hand until Captain America was overpowered by Iron Man, Barnes saved him by knocking Stark back with Captain America's shield, leading to a two on one brawl. Iron Man quickly shot Captain America backward, leaving Winter Soldier to fight Iron Man alone in hand-to-hand combat. Barnes attempted to remove Iron Man's arc-reactor, but the latter used his unibeam to disintegrate the Winter Soldier's prostethic arm.
Captain America rushed at Iron Man in rage to defend Barnes from being executed, leading to Iron Man to shoot beams at the former's shield. Captain America then pushed Iron Man into a wall and began to pummel him, with the latter having the ability to stop the super soldier. Iron Man ordered F.R.I.D.A.Y. to analyze Captain America's fighting pattern, stopping his shield and beat the captain into submission. Rogers again tried to tell Stark that he was defending his friend and that he was his friend, but Stark refused to listen and continued the fight, ordering Rogers to stand down.
Rogers refused and stated he could do this all day, Iron Man's foot was grabbed by a wounded Winter Soldier, who was kicked back. Captain America then grabbed Iron Man from behind, picking him up and slamming him into the ground. Rogers then proceeded to destroy Stark's helmut and the arc reactor, shutting down his suit. Rogers helps Barnes up, but leaves his shield after Stark protests that he does not deserve and that his father made the shield. 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 and Barnes flee to Wakanda, where T'Challa grants them asylum. Barnes decides to go under cryosleep until such a time that he can be mentally cured of his HYDRA brainwashing. Rogers warns T'Challa that if the outside world were to discover that Wakanda was harboring the Winter Soldier, they would come for him, but T'Challa brushes it aside, proudly stating that he would let them try. Rogers then leaves Wakanda and breaks into the Raft, freeing his allies Scarlet Witch, Falcon, Hawkeye and Ant-Man.