Captured by HYDRA, Bucky Barnes was taken to the Siberian HYDRA facility where Arnim Zola replaced his missing limb with a prosthetic arm and turned him into a HYDRA assassin. For the next several decades, Barnes was activated to assassinate anyone who posed a threat to HYDRA. After successfully completing his missions, his mind was erased and he was put into cryostasis to ensure that he would feel remorseless towards those he killed, making him one of the most dangerous assassins in the world.
On December 16, 1991, the Winter Soldier was reactivated by Vasily Karpov and subsequently put under the Memory Suppressing Machine to wipe his memories and emotions. During his brainwashing session, Karpov recited his trigger words from the Winter Soldier Book to activate his brainwashing and training. After the Winter Soldier became obedient, Karpov ordered him to intercept a vehicle carrying a case of the Super Soldier Serum with no witnesses. On his mission in Long Island, New York, Barnes crashed the vehicle and ensured the serum was safe before proceeding to kill Howard and Maria Stark. Afterwards, Barnes returned to the facility and delivered the serum to HYDRA.
At the Siberian Facility, HYDRA assembled five of the most elite assassins for the Winter Soldier Program with the new Super Soldier Serum. After being injected with the serum, the five assassins began transformation without the use of Vita Radiation at the expense of excruciating pain. Afterwards, Karpov put the Winter Soldiers through training with Barnes to test them. During a sparring session between Josef and Barnes, Josef managed to overpower Barnes despite his robotic arm, proving the experiment to be a success.
Suddenly, the Winter Soldiers lost control and became aggressive and hostile to their HYDRA handlers. In response, HYDRA put all of them in Cryostasis Chambers. After the dissolution of the Soviet Union in 1991, the facility was abandoned with the Winter Soldiers still under suspended animation.
Almost 25 years later, Helmut Zemo swore revenge against the Avengers for the death of his family in the Battle of Sokovia, and sought to destroy them. After instigating the Bombing of the Vienna International Centre and the Clash of the Avengers, he led Captain America, Bucky Barnes, and Iron Man to the HYDRA Siberian Facility, under the threat of releasing the Winter Soldiers. However, when the trio arrived, they found that Zemo had murdered all of the Winter Soldiers still under suspended animation. Zemo explained that the Winter Soldiers were merely to motivate them into coming to the facility, and he explained his year-long hatred and obsession with the Avengers for their role in the death of his family.
Zemo then revealed archive surveillance footage of the assassination of Howard and Maria Stark to Iron Man, revealing Bucky to have been his parents' killer. Outraged, Iron Man began to fight Captain America and the Winter Soldier, attempting to kill the latter. Captain America tried to explain that he was under mind control by HYDRA, but he ignored him and blamed him for killing his parents. During their brutal fight, the Winter Soldier tried to pull out Iron Man's arc reactor with his robotic arm, until Iron Man destroyed his arm with his unibeam and incapacitated him. Left alone, Captain America confronted Iron Man, deflecting his repulsor blasts with his shield.
Outside of the facility, Black Panther confronted Zemo, who remarked that he almost killed the wrong man due to Zemo's machinations. Zemo explained that his family was killed during the Battle of Sokovia, and to exact vengeance, he could not outright kill them, but he could manipulate them into killing each other. Then, he apologized for killing his father, who seemed to be a good man who raised a dutiful son. T'Challa realized at how vengeance had consumed Zemo and was consuming Rogers and Stark, and swore off vengeance, deciding to spare Zemo. With his plan successful, Zemo attempted to commit suicide, but T'Challa stopped him and arrested him.
Meanwhile, Rogers continued to fight Stark, who had pinned him to the wall and furiously attacking him with his shield and fist. However, F.R.I.D.A.Y. had quickly analyzed his fight pattern and Iron Man gained the upper hand again, beating him bloody and throwing him across the room. However, Barnes managed to distract Stark, allowing Rogers to beat him before destroying his Arc Reactor, disabling Stark's armor. Exhausted, Rogers left his shield to Stark, who taunted him that it was made by his father, picked injured Barnes and left the facility.