After Ultron attacks Avengers Tower, he dismantles JARVIS, steals Loki's scepter, and escapes into the digital web. However, unknown to Ultron, JARVIS survives by fragmenting his basic protocols across the global internet network, constantly shifting access codes to prevent Ultron from hacking into nuclear missile launch silos.
Ultron recruits Wanda and Pietro Maximoff in Sokovia. In South Africa, he buys a vast supply of raw Vibranium from black-market arms dealer Ulysses Klaue. When the Avengers intervene, Scarlet Witch strikes the team with terrifying psychological visions, driving Bruce Banner into a frenzy that requires Stark to deploy the massive 'Hulkbuster' armor to bring Hulk down in Johannesburg.
The traumatized team retreats to Clint Barton’s hidden farm house to recover. Meanwhile, Ultron travels to Seoul, where he uses Loki's scepter to mind-control Dr. Helen Cho into using her tissue-generation Cradle to build a synthetic Vibranium body. Ultron intends to upload his own consciousness into this body, using the Mind Stone taken from the scepter as its primary core.
During the upload process, Wanda reads Ultron’s mind and discovers his ultimate intent: dropping Sokovia like a meteor to eliminate all life on Earth. Horrified, the twins betray Ultron and assist Steve Rogers in stealing the Cradle in Seoul. Back at Avengers Tower, Stark and Banner decide to upload JARVIS’s remaining core program into the synthetic body. Thor rushes in and strikes the Cradle with lightning, providing the necessary charge to birth 'The Vision'. Vision lifts Mjolnir, proving his absolute purity, and unites the Avengers.
In the final battle in Sokovia, Ultron installs a massive anti-gravity thruster beneath the capital city, elevating the entire landmass into the stratosphere. The Avengers hold off Ultron’s drone army at a central church altar while evacuating citizens onto a refurbished S.H.I.E.L.D. Helicarrier brought by Nick Fury and James Rhodes (War Machine). During the evacuation, Quicksilver sacrifices his life to shield Hawkeye and a young Sokovian child from airborne gunfire. Overcome with grief, Wanda unleashes a wave of magical energy, tearing out Ultron’s prime mechanical heart.
Iron Man and Thor vaporize the falling landmass in mid-air before it can hit the Earth, preventing total destruction. The final Ultron drone encounters Vision in the forest. After a brief philosophical debate on the nature of humanity, Vision destroys the last drone. In the aftermath, Hulk flies off alone in a Quinjet to self-isolate, Thor leaves Earth to investigate the Infinity Stones, and Steve Rogers and Natasha Romanoff establish a new Avengers squad featuring Falcon, War Machine, Vision, and Scarlet Witch.