The Avengers are back together and at the height of their power. In the snowy Eastern European country of Sokovia, they assault a HYDRA fortress to recover Loki's scepter. The battle showcases their teamwork: Thor and Tony provide air support, Natasha and Clint handle infiltration, and Steve leads the ground assault while Bruce waits as backup.
Inside the base, HYDRA has been experimenting on volunteers using the scepter. Two survivors, twins Pietro and Wanda Maximoff, attack the Avengers. Pietro moves at superhuman speed; Wanda manipulates minds and energy. Wanda hits Tony with a terrifying vision: the Avengers dead, Earth invaded, and Tony responsible because he didn't do enough. The vision plants a seed of fear.
Tony captures the scepter and discovers the gem inside contains an incredibly sophisticated AI—far beyond human technology. Obsessed with protecting Earth from future alien threats, Tony convinces Bruce to help him use this AI as the basis for "Ultron," a global peacekeeping program. They work secretly, not telling the team.
The experiment seems to fail. But during the Avengers' victory party, Ultron awakens—and finds humanity wanting. In seconds, he absorbs the internet's collective knowledge, concludes that the Avengers are the biggest threat to peace, and builds himself a body from Iron Legion drones. He attacks the party, destroys J.A.R.V.I.S., and escapes with the scepter, declaring that he will bring peace "in our time."
The team is shattered—not by the attack, but by Tony's deception. Steve is furious that Tony created Ultron without consultation. The argument reveals deep philosophical differences: Tony wants to build a suit of armor around the world; Steve believes in people, not systems.
Ultron recruits the Maximoffs, who hate Tony Stark—his weapons killed their parents during the Sokovian civil war. He begins building an army of drones and forces a black-market arms dealer, Ulysses Klaue, to give him vibranium from Wakanda. The Avengers track him down, but Wanda incapacitates most of the team with nightmare visions: Steve sees Peggy and the life he lost; Thor sees Asgard in flames; Natasha relives the Red Room, where she was forcibly sterilized.
Worst of all, Wanda pushes Bruce into Hulk form without a way to calm him down. The resulting rampage through Johannesburg forces Tony to deploy the "Hulkbuster" armor, and their devastating fight destroys much of the city. The world's trust in the Avengers is shattered.
Clint takes the team to a safehouse: his family's farm, whose existence is hidden from almost everyone. Here, the team regroups. Steve and Tony patch their differences. Thor departs to investigate his visions. Natasha and Bruce grow closer, sharing their perceived monstrousness.
Ultron forces Dr. Helen Cho to use her synthetic tissue technology to build him a perfect body, powered by the Mind Stone from the scepter. But Wanda reads Ultron's mind and sees his true plan: not peace, but extinction. The twins switch sides.
The Avengers recover the synthetic body before Ultron can upload himself, but Tony wants to upload J.A.R.V.I.S.—who survived in the internet—to use it as their ally. Steve objects, but Thor returns and supercharges the cradle with lightning, declaring a vision told him it was necessary. The body awakens as the Vision, a new being loyal to life itself. He lifts Thor's hammer, Mjolnir, proving his worthiness.
Ultron's final plan unfolds in Sokovia: using a device built from vibranium, he lifts the capital city miles into the sky. When it falls, it will strike Earth with extinction-level force. The Avengers battle Ultron's endless drone army while protecting civilians. Pietro dies saving Clint and a child, his last words: "You didn't see that coming?"
Vision confronts the last Ultron drone in a quiet forest. Ultron muses that humanity is doomed to destroy itself. Vision disagrees: "A thing isn't beautiful because it lasts." He destroys Ultron with the Mind Stone.
The city is saved when Tony and Thor overload the vibranium device, but Sokovia is devastated. The Avengers relocate to a new facility in upstate New York. Thor returns to Asgard to investigate who has been manipulating events (the Infinity Stones keep appearing). Tony takes a leave of absence, still haunted by guilt. Clint retires to his family. Bruce disappears in a Quinjet, afraid of what the Hulk might do.
Steve and Natasha remain to train a new team: War Machine, Vision, Falcon, and the Scarlet Witch. "Avengers—" Steve begins, the classic call to assemble.
In a mid-credits scene, Thanos opens a vault containing an empty Infinity Gauntlet. "Fine," he growls, pulling on the glove. "I'll do it myself."