In 1991, a brainwashed super-soldier named James "Bucky" Barnes, portrayed by Sebastian Stan, is sent from a Hydra facility in Siberia on a crucial mission to seize an automobile containing a case of super-soldier serum. Activated through a set of code words inscribed in a Red Book, Barnes becomes an obedient soldier, ultimately leading to a violent operation where he attacks a Cadillac, resulting in the tragic death of its occupants and the theft of the serum from the vehicle's trunk.
Fast forward to the current day, just over a year since the Avengers' confrontation with Ultron in Sokovia. Steve Rogers, played by Chris Evans, along with his fellow Avengers Natasha Romanoff (Scarlett Johansson), Sam Wilson (Anthony Mackie), and Wanda Maximoff (who is portrayed by Elizabeth Olsen), attempt to thwart Brock Rumlow ([Frank Grillo]), who is trying to steal a hazardous biological weapon in Lagos. Despite their best efforts, Rumlow escapes, ultimately detonating a bomb in a desperate bid against the Avengers, revealing that Barnes is not only tied to his schemes but remembers Rogers.
This incident leads U.S. Secretary of State Thaddeus Ross ([William Hurt]) to inform the Avengers about the Sokovia Accords, a petition being prepared by the United Nations that mandates oversight of their operations. The Avengers find themselves split: Tony Stark supports the governance due to his role in the events that led to Sokovia’s destruction, whereas Rogers believes in his moral judgment over governmental control.
Tensions escalate when a bomb kills King T'Chaka ([John Kani]), the father of T'Challa (Chadwick Boseman), at a conference in Vienna aimed at ratifying the accords. Evidence suggests that Barnes is behind the explosion, triggering T'Challa's vow for vengeance. Meanwhile, Rogers is determined to protect his childhood friend Barnes from both the authorities and T'Challa. The two manage to track Barnes to Bucharest, where he vehemently denies involvement in the bombing.
As the plot thickens, Vision attempts to prevent Wanda's departure on orders from Stark, revealing the fracture within the team regarding oversight. Rogers aims to bring Barnes in peacefully but discovers that Stark has kept Wanda confined under the pretense of safety, leading to heightened conflict.
Helmut Zemo ([Daniel Brühl]), a cunning antagonist, locates and murders Barnes' former Hydra handler, seizing the book that contains activation phrases for Barnes’ programming. He manipulates Barnes into a rampage, intending to escape, and reveals that he orchestrated the Vienna attack. Understanding this new threat, Rogers opts to break the rules and enlists allies like Clint Barton (Jeremy Renner), Wanda, and Scott Lang in order to stop Zemo.
With permission from Ross, Stark assembles his team including Romanoff, T'Challa, James Rhodes (Don Cheadle), Vision, and a newly updated Peter Parker (Tom Holland) to capture the renegade team. A confrontation ensues at Leipzig/Halle Airport, where, despite the heated battle, Romanoff chooses to aid Rogers and Barnes in their escape.
After discovering Zemo's manipulation and the truth behind Barnes' alleged crimes, Stark finds Rogers and Barnes in Siberia just as Zemo reveals that he has killed other super-soldiers. In a heartbreaking confrontation, Stark learns of Barnes' history with his parents, leading to a fractured relationship among the heroes. Enraged, he attacks both Rogers and Barnes, which culminates in further devastation.
After Zemo’s plans crumble, he is apprehended, while Stark aids Rhodes in regaining the ability to walk through exoskeleton technology. A significant moment arises as Rogers orchestrates a daring breakup of his captured friends from prison.
In a poignant mid-credits scene, Barnes is given asylum in Wakanda, agreeing to return to cryogenic stasis until he can be freed from his conditioning. Meanwhile, in a post-credits tease, Parker experiments with a new gadget, hinting at more adventures to come.
