The film opens with a 1991 flashback showing a brainwashed Winter Soldier forcing a car off the road and stealing super-soldier serum vials. In the present day, Helmut Zemo bombs the UN assembly in Vienna, killing King T'Chaka, and frames Bucky Barnes. Zemo recites trigger words in Berlin to trigger Bucky's rampage and question him about the 1991 mission, heading to a Siberian base where five other cryogenically frozen Winter Soldiers reside.
Cap recruits Falcon, Scarlet Witch, Hawkeye, and Ant-Man to stop them, while Tony recruits War Machine, Black Widow, Vision, Black Panther, and Spider-Man to intercept them at Leipzig airport. Following an epic battle, Cap and Bucky escape to Siberia in a Quinjet while their teammates are arrested and sent to the Raft prison.
In Siberia, Tony learns Zemo framed Bucky and flies to assist Cap. However, Zemo reveals his true motive: he shot the other Winter Soldiers in their sleep and instead plays security footage from December 16, 1991, revealing that Bucky brutally beat Howard and Maria Stark to death. Learning Steve concealed this truth, Tony attacks in a fury. Bucky loses his metal arm, Cap disables Iron Man's arc reactor, surrenders his shield, and walks away with Bucky. T'Challa stops Zemo from committing suicide to face formal justice, while Steve breaks his allies out of the Raft and sends Tony a letter promising help whenever needed.