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Sam Wilson has taken up the shield and is now the official Captain America. With the new Falcon, Joaquin Torres, he opens the movie in Mexico, recovering cargo stolen by the mercenary group Serpent, led by Sidewinder. The mission works, but the leader gets away. Soon after, Sam gets a surprising invitation. Thaddeus Ross, the old general who used to hunt the Hulk, is now President of the United States, and he wants both of them at the White House.
Ross has a big pitch. In the middle of the Indian Ocean sits Celestial Island, the petrified body of the Celestial Tiamut. Inside it, people found adamantium, a metal tougher than vibranium. Ross wants a global treaty to share the mining, and he still dreams of putting the Avengers back together. Sam brings along Isaiah Bradley, the forgotten super-soldier the government locked up and experimented on for decades. During the summit, a song plays and a few guests suddenly change. Isaiah is one of them. They open fire on Ross, and Isaiah ends up in a cell as if he were guilty.
Sam doesn't buy the official story and starts digging on his own. He captures Sidewinder, finds a secret prison called Camp Echo One, and lands on a name: Samuel Sterns. Sterns is a scientist who gained off-the-charts intelligence after exposure to Bruce Banner's blood. Ross kept him locked away for years, using that brain. With the song as a trigger, Sterns controls people from afar. At Celestial Island he makes an American pilot attack the Japanese fleet, and Sam and Torres have to race to stop a war. Torres comes out of it badly hurt.
Sterns's real ace is personal. Ross's heart is failing, and he only stays on his feet thanks to pills that Sterns himself made. They were loaded with gamma radiation. When the scheme goes public and Ross loses control, he turns into the Red Hulk and wrecks part of the White House. Sam takes on the monster with almost no chance and only calms him down by talking about Betty, the daughter Ross pushed away, and the cherry blossoms they used to visit together.
In the end, Sterns is locked up, Isaiah is cleared, and Torres gets a spot on a new Avengers lineup that Sam agrees to lead. The adamantium treaty is signed. Ross resigns and turns himself in at the Raft, where Betty comes to visit. The post-credits scene, with Sterns, warns that danger is coming from other worlds, and that comes back later.