Captain America: Civil WarSave

We open in 1991. A snow-blanketed road in Siberia. The Winter Soldier — Bucky Barnes under HYDRA's control — ambushes a car carrying a young Howard and Maria Stark. He kills both with his bare hands, retrieves a case of super-soldier serum vials from the trunk, and disappears. The footage exists on a single hidden tape that no one in the modern day has yet seen. The film will spend two and a half hours bringing that tape into Tony Stark's hands. Cut to: 2016, Lagos. Cap leads a four-person Avengers team — Black Widow, Falcon, Scarlet Witch — to capture a HYDRA splinter cell stealing a bioweapon. The operation goes well. Until Wanda, panicking, accidentally detonates a suicide vest inside an adjacent building. Eleven Wakandan aid workers die.

Secretary of State Thaddeus Ross convenes the Avengers at the upstate compound and presents them with the Sokovia Accords — a UN treaty that would place the team under direct international oversight. The team splits immediately. Tony Stark, haunted by the Sokovian devastation he caused with Ultron and confronted that morning by a grieving mother whose son died in the rubble, supports the Accords. Steve Rogers, who has spent the last two years watching SHIELD collapse into HYDRA in The Winter Soldier, refuses. He has lost trust in every institution. The split is not friendly.

At a Vienna conference where the Accords are being formally signed, a bomb hidden in the venue kills King T'Chaka of Wakanda. Surveillance footage points to the Winter Soldier as the bomber. Bucky has been laying low in Bucharest, but Romanian special forces and CIA agents corner him in his apartment. Steve arrives first to extract him, and the resulting chase across Bucharest's rooftops draws in Black Panther — T'Chaka's son T'Challa, in his first MCU appearance — vowing personal vengeance. Steve, Bucky, and T'Challa are all arrested and brought to a Berlin black site. There, a man named Helmut Zemo activates the Winter Soldier's HYDRA conditioning over the intercom: 'longing, rusted, seventeen...'

Bucky breaks loose, programmed and lethal. Steve and Sam barely contain him. They take him to a safe house. Bucky, finally lucid, tells Steve what Zemo knew: there are five other captive HYDRA-programmed super-soldiers in a Siberian bunker. Steve assembles his own team — Sam, Wanda, Hawkeye, Ant-Man — to find them before Zemo unleashes them. Tony begins assembling his own team to bring Steve in. He recruits Vision, Black Widow (reluctantly), Black Panther, War Machine, and — in a memorable detour to Queens — fifteen-year-old Peter Parker, whom Tony has secretly been monitoring for months. Tony catches Peter at home, gives him a tech-upgraded suit, and brings him to Berlin.

The two teams meet at the Leipzig-Halle Airport. The resulting battle is the most spectacular team-up moment in pre-Endgame MCU history. Ant-Man becomes Giant-Man for the first time. Spider-Man fights Cap with web-bound enthusiasm. Black Panther chases Bucky. War Machine is critically wounded — paralyzed from the waist down — by friendly fire from Vision. The fight ends without a clear winner, but Steve and Bucky escape on the Quinjet and fly to the remote Siberian HYDRA base. Tony, learning that Zemo masterminded the entire conflict to fracture the Avengers, joins them — leaving his team behind, knowing he and Steve must work together.

The three arrive at the Siberian bunker. Zemo has already disabled the other five captive super-soldiers — shooting each one in cryosleep — and has been waiting for Tony specifically. He plays the 1991 tape. Tony watches his parents die at Bucky's hands. He sees Howard Stark recognize the assassin. He realizes Steve has known about this for the better part of a year and hasn't told him. Tony loses control. He attacks Bucky directly, intending to kill him. Steve intervenes. The film's final fight is brutal, intimate, and entirely without quips: Steve and Bucky against Tony in the cramped concrete vault.

Steve disables Tony's chest reactor with his shield, slamming it directly into the arc reactor at point-blank range. Tony, defeated and broken, watches Steve and Bucky leave the bunker without saying anything. Steve drops the shield — the literal symbol of Captain America — on the bunker floor as he leaves. Tony picks it up and looks at it for a long moment. He's now leaderless, friendless, exhausted, and grieving. He flies home. The film closes with him reading a letter from Steve, sent weeks later: 'If you need me, I'll be there.' The flip phone Tony will use to call Steve from Infinity War's opening scene has just arrived.

The post-credits scene introduces Spider-Man clumsily testing his new web-shooters in his Queens bedroom — a soft launch for the Homecoming (2017) rollout. The mid-credits scene shows Bucky in cryosleep at T'Challa's lab. Civil War is, structurally, the moment the MCU permanently lost its post-Avengers cohesion. Every subsequent Phase 3 film — Spider-Man: Homecoming, Black Panther, Infinity War, Endgame — exists in the wake of what Civil War split apart. The team would not be whole again for five years.