Somewhere in Siberia in 1991, Hydra

Somewhere in Siberia in 1991, Hydra revives Bucky Barnes from his decades-long sleep and converts him into the Winter Soldier, equipping him with a cybernetic left arm and controlling his mind with a combination of Russian words. The Winter Soldier becomes Hydra's silent assassin throughout the years.

During the present day, Captain America, Falcon, Black Widow, and Scarlet Witch stop a terrorist group led by Brock Rumlow in Lagos, Nigeria, from taking a biological weapon. Tragedy strikes when Scarlet Witch saves Captain America from Rumlow's suicide bomb, but accidentally directs the explosion toward an office building, killing several Wakandan relief aid workers. Meanwhile, after giving a speech at MIT, Tony Stark is confronted by a woman whose son was killed during the Sokovia incident. Back at their headquarters, the Avengers are paid a visit by Secretary of State Thaddeus "Thunderbolt" Ross, who informs them that the United Nations is in the process of passing the "Sokovia Accord", a system of accountability and a governing body to regulate the team's activities. The team is completely divided over the act while Black Widow flies to Vienna, Austria, for the accord's official signing. The event is marred by a terrorist bombing that kills King T'Chaka of Wakanda. When security cameras reveal the culprit as the Winter Soldier, T'Chaka's son T'Challa swears revenge on him while Captain America and Falcon decide to find Bucky and take him in themselves. Following a tip-off and a grueling chase, Bucky is captured, but Captain America, Falcon, and T'Challa are arrested for obstruction of justice.

While in captivity, Bucky is released by Helmut Zero, who uses Hydra's word hypnotism to send him on a murderous rampage. Captain America and Falcon manage to stop him and flee out of sight. After coming to his senses, Bucky tells Captain America that Zero is headed for Siberia to release a group of Super Soldiers for his use. Iron Man assembles his team with Black Widow, War Machine, Vision, Black Panther, and Spider-Man to capture Captain America's team, which also consists of Hawkeye, Scarlet Witch, and Ant-Man. Both factions duke it out at Leipzig/Halle Airport until Captain America and Bucky commandeer a Quinjet and head for Siberia. War Machine is seriously injured while chasing the Quinjet and Vision accidentally shoots him down. Later, Stark discovers through surveillance files that the Winter Soldier was framed by Zero and learns of Captain America and Bucky's destination through an imprisoned Falcon.

Iron Man arrives in Siberia and has a truce with Captain America and the Winter Soldier, but the trio discover that the Super Soldiers have been murdered by Zero, who reveals to them that he was a Sokovian and his only closure to the loss of his country is the self-destruction of the Avengers. Zero then plays a security video from December 16, 1991, where the Winter Soldier assassinated Stark's parents to acquire five samples of the Super Soldier serum from their car. An enraged Iron Man then battles both Captain America and the Winter Soldier until he breaks the Winter Soldier's bionic arm before Captain America destroys his Arc Reactor with his shield. Stark warns Rogers that his shield belongs to the U.S. government, to which Rogers drops it before taking Bucky away with him. Meanwhile, after witnessing Zero's motives, Black Panther stops him from committing suicide and brings him to justice. Days later, Captain America springs his comrades from a maximum security prison in the Atlantic Ocean and they take refuge in the Kingdom of Wakanda. Knowing the consequences of his past, Bucky has himself placed in a cryogenic sleep.