Captain America: Civil War

After Sokovia and a botched operation in Lagos where Wanda's containment of a bomb kills Wakandan aid workers, 117 nations draft the Sokovia Accords: enhanced individuals must operate under UN authority or retire.

Stark signs, driven by guilt — he created Ultron. Rogers refuses, having watched S.H.I.E.L.D. turn out to be HYDRA and concluding that institutions cannot be trusted with this power. Both positions are defensible. That's the film's actual achievement.

If Wakanda was secret, how was T'Chaka at a UN signing? Because Wakanda was never hidden as a country. It had a seat at the UN, an ambassador, and a king who attended summits. What it hid was its technology and wealth — it presented as a poor agrarian state. So T'Chaka attending Vienna is ordinary diplomacy. What T'Challa reveals at the end of Black Panther is that the poverty was a front. Different secret.

At the signing in Vienna, a bomb kills King T'Chaka of Wakanda. Bucky Barnes is framed. T'Challa pursues him for vengeance. The real culprit is Helmut Zemo, a Sokovian intelligence officer whose family died in Ultron's attack, whose entire plan is to break the Avengers from the inside rather than beat them.

Zemo's method: he shows Stark grainy footage of 16 December 1991 — Bucky Barnes killing Howard and Maria Stark. Rogers admits he knew. Stark tries to kill Bucky; Rogers stops him; the friendship ends in a bunker in Siberia. Rogers leaves the shield behind.

Team Stark: Stark, Rhodes (paralysed in the fight), Vision, Romanoff (who defects mid-battle), T'Challa, and a recruited 15-year-old Peter Parker. Team Rogers: Rogers, Barnes, Wilson, Wanda, Barton, Lang. The losers are imprisoned in the Raft; Rogers breaks them out. Barnes goes into voluntary cryosleep in Wakanda while T'Challa's scientists work on undoing his conditioning.

The Raft is a maximum-security floating prison for enhanced individuals, in international waters — no coordinates given. Remember it; Ross ends up in it himself in 2025.

What conditioning, and why freeze him? HYDRA implanted ten trigger words in Russian. Anyone who reads the sequence aloud overrides Bucky's personality entirely and turns him into a compliant weapon, with no control on his part. He asks to be frozen because he considers himself a loaded gun until it's gone.

Shuri's team removes it before Infinity War; The Falcon and the Winter Soldier confirms it on screen when Zemo reads the words aloud and nothing happens.

The Avengers do not reassemble before Thanos arrives. That is the point of this film.