- "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
Helmut Zemo located Vasily Karpov, one of the former leaders of the Winter Soldier Program, who was in hiding with HYDRA having been wiped out. Having discovered Karpov's location, Zemo knew that he would be unable to easily break into Karpov's home so instead faked a car crash and asked Karpov to invite him inside to discuss the matter in private.
As soon as Karpov opened the door, Zemo attacked and subdued Karpov before tying him upside down above a sink slowly filling with water. Using a hammer to bust through the walls of Karpov's cellar, Zemo found and took the Winter Soldier Book from Karpov's HYDRA files and introduced himself while speaking politely to Karpov, before demanding information that occurred back in the year 1991. Despite Zemo making his intentions very clear that he would not stop until he had the information he needed to exact his violent revenge, noting that his loyalties did not lie within HYDRA, Karpov refused to help him. Seeing that Karpov would not speak to him, Zemo left the tap running while Karpov gave the famous HYDRA salute. Zemo then walked straight out of the house, taking his new stolen belongings.
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. 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.
After enlisting Clint Barton to rescue Wanda Maximoff and recruit Scott Lang, who had been recommended by Wilson, Rogers and his team made their way to Flughafen Leipzig-Halle, where a Quinjet was waiting to take them to Siberia. Iron Man evacuated the airport, alerting Captain America and his team to the heroes' presence. Captain America jogged out onto the runway towards a helicopter, which was then quickly disabled by Iron Man as he, War Machine, Black Panther and Black Widow approached Captain America. Captain America attempted to convince them that Barnes was innocent, however Iron Man grew impatient and ordered Spider-Man to web Captain America's hands and steal his shield.
Falcon soon located the Quinjet and notified Captain America, who then gave the signal to Hawkeye. Hawkeye fired an arrow at the webbing, freeing Captain America as Ant-Man revealed himself to be hiding on Captain America's shield and managed to take Spider-Man by surprise, hitting him and retaking the shield. Stark turned around to the source of the arrow, locating Hawkeye and Scarlet Witch via scan as War Machine announced Falcon and Barnes' position and the battle began.
Before disappearing from the airport, Natasha Romanoff told Stark that Barnes was framed by Helmut Zemo, and Stark later found evidence confirming this. Stark followed Rogers and Barnes to the HYDRA Siberian Facility, and was shadowed by T'Challa. They discovered that the other Winter Soldiers had been killed by Zemo, who is Sokovian and wanted to punish the Avengers for his family's death. He showed them footage revealing the Winter Soldier killing Stark's parents in 1991. Rogers held back a furious Stark, who then interrogated him on his knowledge of Barnes' involvement in his parents' death. Rogers denied it, but Stark saw through the lie, forcing Rogers to admit he'd known all along. Horrified to learn that a man he considered a friend had been lying to him for years, Stark took a step back.
Enraged, Stark turned on his former friend, reengaging his helmet and knocking Rogers to the ground. As Zemo retreated, Stark attacked Barnes and quickly overpowered him, destroying his assault rifle and pinning him to the floor. Rogers threw his shield, deflecting an otherwise fatal repulsor blast, but Stark repelled Rogers before restraining his ankles. He then resumed his assault on Barnes, pinning him against a wall and charging up his repulsor, but Barnes grabbed hold of it and manages to disable the repulsor gauntlet with his Prosthetic Arm. Stark once again tried to kill Barnes, this time with a missile at point-blank range, but Barnes deflected it, resulting in the destruction of one side of the chamber and pinning Stark beneath rubble. At Rogers' urging, Barnes then took his chance and ran for an escape hatch located at the top of the facility. Rogers destroyed his restraints and tried to talk Stark down, to no avail. Stark tried to avoid fighting Rogers by simply flying past him, but Rogers grabbed him by the ankle and destroyed his left boot jet with his shield, compromising his flight systems. Before he could do anything else, however, he was blocked in by falling rubble, courtesy of Stark's laser beam.
Stark continued his pursuit of Barnes, firing a repulsor blast at him, only for Rogers to bounce it back at him with the shield, sending Stark plummeting to the floor below. Rogers used a grappling hook to drag Stark further down while Barnes managed to reach the hatch at the top of the silo. Rogers threw his shield again, but Stark blasted it aside and aimed a missile at Barnes. Failing to get a lock-on, Stark aimed with his own eyes and fired the missile, destroying the hatch and trapping Barnes inside the silo.
Barnes attempted to defend himself with a metal pipe, but Stark got him in a headlock and began strangling him, questioning if he remembered everyone he killed for HYDRA. Rogers then tackled Stark, causing all three of them to plummet over a dozen stories to the bottom of the silo. Rogers futilely attempted to talk Stark down, but the latter was determined to avenge his parents' deaths.
The two begin brawling, and Stark eventually gains the upper hand. However, Barnes takes up the shield and intervenes, leading to a 2-on-1 fight. Rogers took a repulsor blast to the chest that sent him flying across the room, while Barnes and Stark continued to fistfight. Barnes gained the upper hand, pinning Stark to the wall and trying to rip out his arc reactor, but this backfired spectacularly, as Stark used the uni-beam to vaporize his prosthetic arm before hitting with a repulsor blast at point-blank range.
Enraged, Rogers leaped to his feet and charged towards Stark, determined to protect his best friend. Rogers quickly overpowered Stark and battered him relentlessly. Realizing he was losing the fight, Stark ordered F.R.I.D.A.Y. to analyze Rogers' fighting pattern. With the countermeasures prepared, Stark blocked a shield blow from Rogers and turned the tables on him, beating him into submission. Facing defeat, Rogers once again tried to talk sense into his former friend, but Stark bitterly responded that they were enemies now before tossing Rogers aside.
Stark ordered Rogers to stand down for the final time, but true to his nature, Rogers struggled to his feet and raised his fists, stating that he could do this all day. Stark charged up a repulsor blast, but was distracted when Barnes grabbed him by the boot. Stark turned and violently kicked Barnes in the face, but left himself open to Rogers, who grabbed him from behind and hoisted him over his head. Rogers violently threw Stark to the ground before punching off his helmet.
Taking up his shield, Rogers brought it down hard, burying it deep in Stark's chestplate, cleaving his arc reactor in two and disabling his armor. Taking an emotional moment to catch his breath, Rogers removed the shield and helped Barnes to his feet. As the two walk away, Stark protested that Rogers didn't deserve the shield his father had made. After a moment's hesitation, Rogers dropped the shield. Satisfied that he had irreparably fractured the Avengers, Zemo attempted suicide, but was apprehended by T'Challa.
In the aftermath, T'Challa handed Zemo over to Everett Ross, who placed Zemo under custody. Stark provided Rhodes with exoskeletal leg braces to let him walk again. Rogers and Barnes escaped to Wakanda, where T'Challa granted them asylum. Barnes decided to go under cryosleep until such a time that he can be mentally cured of his HYDRA brainwashing. Rogers warned 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 brushed it aside, proudly stating that he would let them try. Rogers then left Wakanda and broke into the Raft, freeing Scarlet Witch, Falcon, Hawkeye and Ant-Man.
- Avengers: Endgame Prelude (mentioned)