The Avengers tracked Loki's Scepter to a HYDRA Base in Sokovia. The team, consisting of Iron Man, Captain America, Thor, Hulk, Black Widow, and Hawkeye, raid the outpost, fighting Chitauri enhanced soldiers and tracking down Wolfgang von Strucker, who had been experimenting with human enhancement. Two of Strucker's experiments, Pietro and Wanda Maximoff tried to fight the Avengers but Captain America apprehended Strucker, while Stark retrieved the scepter.
Stark analysed the Scepter and discovered an artificial intelligence within the gem. He showed Banner and used the AI to complete Stark's "Ultron" global defense program. The unexpectedly sentient Ultron, believing he must eradicate humanity to save Earth, eliminated J.A.R.V.I.S. and attacked the Avengers during a victory party at their headquarters. Escaping with the scepter, Ultron used the resources in Strucker's Sokovia base to upgrade his rudimentary body and build an army of robot drones. He recruited the Maximoff twins, who harbored a grudge against Stark because his technology was responsible for their parents' deaths, and together, they visited the South African shipyard base of arms dealer Ulysses Klaue to obtain vibranium. The Avengers battled the twins, but Wanda subdued them with haunting visions, causing the Hulk to run amok and forcing Stark to use his powerful Hulkbuster armor to stop him.
A worldwide backlash and the doubts and fears Wanda's hallucinations had caused sent the team into hiding at Barton's safehouse farm, where they meet Barton's wife, Laura, and their children. Thor departed to consult with Dr. Erik Selvig on the meaning of the apocalyptic future he saw in his hallucination. Realizing an attraction between them, Romanoff and Banner planned to flee together after fighting Ultron. Nick Fury arrived and encouraged the team to form a plan to stop Ultron. In Seoul, South Korea, Ultron forced Banner's friend Dr. Helen Cho to use her synthetic tissue technology together with the scepter's gem to create the perfect body for him. When Ultron began uploading himself into the body, Wanda was able to read his mind; discovering his plan for human extinction, the Maximoffs turned on Ultron. Rogers, Romanoff and Barton hunted Ultron and retrieved the synthetic body, but Ultron captured Romanoff in the process.
The Avengers fought amongst themselves when Stark secretly uploaded J.A.R.V.I.S., who was still operational after hiding inside Ultron on the internet, into the synthetic body. Thor returned to help activate the body with lightning, explaining that the gem on its brow was one of the six Infinity Stones, the most powerful objects in existence and was part of his "vision". Taking "Vision" as its name, the robot and the Maximoffs joined the Avengers, and the team headed for Sokovia, where Ultron had used the vibranium to upgrade himself and build a machine to lift a large part of the city skyward, intending to crash it into the ground to cause global extinction. As the city began to lift, Banner rescued Romanoff, who awakened the Hulk for the battle. The Avengers fought Ultron's army while delaying Ultron from activating his plan's final procedure. Fury arrived in a Helicarrier with Maria Hill, James Rhodes and S.H.I.E.L.D. agents to assist in evacuating civilians, but Pietro was killed when he saved Hawkeye from a barrage of fire. A grieving Wanda abandoned her post to destroy Ultron's primary body in revenge, inadvertently allowing one of his drones to deactivate his machine. The landmass plummeted, but Iron Man and Thor overloaded the machine and shattered the city into pieces. In the aftermath, the Hulk, unwilling to endanger Romanoff by being with her, departed in a Quinjet, while the Vision confronted Ultron's last remaining body and destroyed it.
Later, the Avengers have established a new base in upstate New York, run by Fury, Hill, Cho, and Selvig. Fury assured Romanoff that Banner likely escaped the crashed Quinjet. Believing the Mind Stone was safe with the Vision, Thor returned to Asgard to learn more about the forces he suspected had manipulated recent events. Stark and Barton retired from the team, and Rogers and Romanoff prepared to train new Avengers: Rhodes, Wanda Maximoff, Vision and Sam Wilson.