The full story
Sam Wilson is Captain America, and the world won't let him forget he's not Steve Rogers. No super-soldier serum. No shield from the 1940s—this one was made by Wakandans as a gift, and some see it as a participation trophy. He's invited to the White House for a summit: the U.S., Japan, and other nations negotiating a treaty over a massive adamantium deposit discovered inside Celestial Island, the remains of the dead Celestial from Eternals.
It's a trap. Not Ross's trap specifically, but someone is manipulating events to destabilize the new administration. At the summit, Isaiah Bradley—the forgotten super-soldier from the 1950s who was imprisoned and experimented on for decades—is secretly triggered by a hidden cue and opens fire, seemingly at Ross. He's arrested immediately. Sam knows Isaiah would never do this and sets out to prove he was used.
The Leader returns. Samuel Sterns, last seen in The Incredible Hulk with gamma radiation mutating his brain, has spent years in government custody—used by Ross as a weapons-research asset in exchange for a freedom that never came. His plan is revenge: frame Bradley for the assassination attempt, push the adamantium treaty toward a US-Japan war, and destroy Ross personally and politically.
Ross has a secret of his own: a terminal heart condition, treated with medication he's been getting from Sterns. The pills are secretly laced with gamma radiation, accumulating in his body for months. When the conspiracy reaches him live, in front of the press, the rage detonates it. He transforms. The Red Hulk is rage without the balance of Banner's intelligence—pure military fury embodied.
Sam has to fight the President of the United States while trying not to kill him. No serum means every hit from Red Hulk could be fatal. The shield can only do so much when you're a normal human fighting a gamma-powered monster. The fight tears through the cherry blossom trees around the White House until Sam reaches him—not with force, but by invoking his daughter Betty, reminding Ross who he still is underneath the rage.
Sterns gets his revenge and turns himself in—he's sent to the Raft. Isaiah is exonerated. Ross, shaken but still president, brokers a treaty over the adamantium rather than let it start a war.
The film ends with Sam considering restarting the Avengers. The world needs them again, and this time they won't be beholden to governments or accords. They'll answer to the people who need help.
The post-credits scene shows Sterns imprisoned but smiling, telling Sam through the glass that the multiverse is unstable and other worlds might invade soon. The Leader sees every branching timeline, and something is coming.