Steve Rogers runs. Every morning, lap after lap around the National Mall, he runs—because sleep brings nightmares of the ice, of Peggy, of a world that moved on without him. He's working for S.H.I.E.L.D. now, leading strike teams on missions, but he's increasingly uncomfortable with the organization's methods. Director Nick Fury trusts him, but trust is relative when your employer keeps secrets.
During a mission to rescue hostages from a S.H.I.E.L.D. ship, Steve discovers that Natasha Romanoff had a separate, secret objective: extracting data from the ship's computers. Furious at being kept in the dark, Steve confronts Fury. In response, Fury shows him Project Insight: three next-generation Helicarriers designed to preemptively eliminate threats using an algorithm that predicts hostile behavior. The carriers will hover indefinitely, connected to targeting satellites, able to kill thousands per minute.
Steve is horrified. "This isn't freedom. This is fear."
Fury begins to share Steve's concerns and attempts to delay Project Insight, but he's attacked on the streets of Washington by a tactical team led by a figure with a metal arm: the Winter Soldier. Fury barely escapes to Steve's apartment, where he warns Steve that S.H.I.E.L.D. is compromised before being shot through the wall by the Winter Soldier. Fury dies in surgery, but not before slipping Steve a flash drive containing the ship's data.
Steve is summoned by Alexander Pierce, a senior S.H.I.E.L.D. official and World Security Council member, who demands information about Fury's death. Steve refuses to share what he knows. Minutes later, he's branded a fugitive, attacked in an elevator by an entire S.H.I.E.L.D. strike team. He fights his way out and escapes with Natasha's help.
The flash drive leads them to an abandoned S.H.I.E.L.D. bunker in New Jersey—the same camp where Steve trained during the war. Hidden beneath the building, they find something impossible: Arnim Zola, the HYDRA scientist Steve fought in World War II, his consciousness preserved in an ancient computer system. Zola reveals the terrible truth: HYDRA was never destroyed. After the war, S.H.I.E.L.D. recruited Zola and other HYDRA scientists through Operation Paperclip. They've been growing inside S.H.I.E.L.D. ever since, manipulating world events, including Howard Stark's assassination.
Project Insight is HYDRA's endgame: eliminate millions of potential threats—anyone who might oppose their new world order—in one surgical strike. The algorithm doesn't predict hostile behavior; it identifies free thinkers, leaders, anyone who won't submit.
Steve and Natasha barely escape a missile strike and seek refuge with Sam Wilson, a veteran and counselor Steve befriended while running. Sam has access to a prototype military wingsuit: the Falcon. The three of them capture a HYDRA agent who reveals that the Winter Soldier is the key to Project Insight's security.
Steve finally confronts the Winter Soldier and is shattered to recognize him: it's Bucky Barnes. HYDRA recovered Bucky after his fall, replaced his lost arm with a cybernetic weapon, and subjected him to decades of brainwashing and cryogenic freezing between missions. Bucky doesn't recognize Steve at all.
The final battle erupts at the Triskelion, S.H.I.E.L.D.'s headquarters, as the Helicarriers launch. Steve and Sam fight to replace targeting chips that will turn the carriers against each other while Natasha exposes HYDRA's secrets to the world, dumping every classified S.H.I.E.L.D./HYDRA file onto the internet—including her own dark history. Agent Maria Hill coordinates from inside, and even the World Security Council turns on Pierce.
Steve boards the final Helicarrier, confronting Bucky. Even as Bucky beats him nearly to death, Steve refuses to fight back. "I'm not gonna fight you. You're my friend." He drops his shield. "Then finish it. Because I'm with you 'til the end of the line."
The words—their shared promise from childhood—break through Bucky's programming. When Steve falls from the crashing Helicarrier into the Potomac, Bucky dives in and saves him, then disappears.
S.H.I.E.L.D. is destroyed, HYDRA exposed, and the world changed forever. Steve survives, and with Sam's help, begins searching for Bucky.
In a mid-credits scene, HYDRA scientists study Loki's scepter while two prisoners—"the twins"—display impossible abilities: one moves at superhuman speed, the other manipulates objects with her mind. They are Pietro and Wanda Maximoff.
In a post-credits scene, Bucky visits the Smithsonian exhibit about Captain America, staring at the display about himself, searching for memories of who he used to be.