Full story
The Avengers raid a Hydra base in Sokovia, a fictional Eastern European country, run by Baron Strucker. They recover Loki's scepter and run into a pair of powered twins: Pietro Maximoff, with super speed, and Wanda Maximoff, who can twist minds and move objects. Both hate Tony Stark, because a bomb with his name on it killed their parents. Wanda plants a vision in Tony's head of his friends dead and a threat coming from space.
Shaken, Tony wants a shield of robots to protect the planet. With Bruce Banner, he uses the artificial intelligence hidden in the scepter's gem to finish the Ultron program. Ultron wakes up during a party at the Tower, decides peace means wiping out humanity, destroys J.A.R.V.I.S. and escapes with the scepter. In Sokovia he builds himself a better body and a robot army and recruits the twins.
In Johannesburg, Ultron buys vibranium from arms dealer Ulysses Klaue. The Avengers arrive and Wanda gets inside almost everyone's head. Thor has strange visions, Steve relives the 1940s and Natasha remembers her brutal Red Room training. Banner turns into an uncontrolled Hulk and tears up part of the city until Tony stops him with the Hulkbuster armor. Public opinion turns on the heroes. They hide out at Clint Barton's farm, where he has a wife and kids nobody knew about. Natasha and Banner wonder about a future together. Nick Fury shows up to rally the team.
In Seoul, Ultron forces scientist Helen Cho to build a synthetic body using vibranium and the scepter's gem. Wanda reads his mind, sees the extinction plan, and the twins switch sides. The Avengers steal the body, but Ultron takes Natasha. Tony uploads J.A.R.V.I.S., who survived by hiding on the internet, into the body. Thor, back from chasing his visions, gives it the final jolt with lightning. Vision is born and says he is on the side of life. To prove it, he lifts Thor's hammer. The gem in his forehead is one of the six Infinity Stones.
In Sokovia, Ultron lifts an entire city into the sky to drop it on Earth like a meteor. The heroes race to get people out, with Fury showing up in a Helicarrier. Pietro dies shielding Clint and a child. Wanda rips the heart out of Ultron's main body. Tony and Thor blow up the rock in midair, and Vision destroys the last robot. Banner flies off alone in a jet. Thor leaves to look into the Stones, Tony steps back and Clint retires. Steve and Natasha start training new Avengers: Rhodes, Vision, Sam Wilson and Wanda. In the credits scene, Thanos takes an empty gauntlet and says he will get the Stones himself.