Avengers: Age of Ultron

Stark and Banner secretly use the Mind Stone in Loki's recovered scepter to build a global defence AI. It wakes up, concludes that the only way to achieve peace on Earth is human extinction, and calls itself Ultron.

Ultron recruits twins Wanda and Pietro Maximoff — Sokovian volunteers experimented on by HYDRA with the Mind Stone, giving Wanda reality-warping abilities and Pietro superspeed.

How Vision is actually created, and why. Ultron wants a body of organic-synthetic tissue — vibranium plus a regeneration "cradle" — so he can be indestructible. The Avengers steal it mid-upload. Stark and Banner want to put J.A.R.V.I.S. in it; Rogers objects violently, having just watched them create Ultron by exactly this method. Thor settles the argument by striking the cradle with lightning.

What emerges is neither Ultron nor J.A.R.V.I.S. but a new being: Vision, with the Mind Stone in his forehead. The proof he's trustworthy is that he casually picks up Mjolnir and hands it to Thor mid-conversation.

Where the Mind Stone came from originally is never explained on screen.

Thanos acquired it before 2012 and gave it to Loki inside the scepter to fund the Chitauri invasion, expecting both it and the Tesseract back afterwards. Loki failed; the scepter went to S.H.I.E.L.D., then quietly to HYDRA, who used it on the Maximoffs.

Ultron lifts the city of Sokovia into the sky to drop it as an extinction meteor. The Avengers evacuate and destroy it. Pietro Maximoff dies. Thousands of Sokovian civilians die in the collateral damage. That civilian death toll becomes the direct legal justification for the Sokovia Accords.

The team splinters. Thor leaves to investigate the Stones. Stark steps back. Rogers and Romanoff train a new roster: Wilson, Rhodes, Wanda, Vision.

Hulk and Hawkeye both effectively leave the story here, which is why they're near-absent from Civil War.

Hulk flies a Quinjet into deep space to remove himself as a threat, crashes on Sakaar, and spends two years there as a gladiator, never once turning back into Banner. He's next seen in Thor: Ragnarok (film, 2017). He misses the Accords entirely.

Hawkeye retires to his hidden farm with Laura and their children. He is in Civil War — Rogers calls him out of retirement to break Wanda out of the compound — but he's imprisoned on the Raft, then takes a plea deal and house arrest, which is why he's absent from Infinity War.