[pr0xyt0xin]

Baron Zemo gathers the strongest Avengers villains who are still in commission.
Crossbones (introduced as Zemo's main man in Cap2)
Enchantress (introduced in Thor3)
Executioner (introduced in Thor3)
Abomination (still captive from TIH)
Radioactive Man (introduced in IM3)

Zemo concocts a plan to enslave the human race and become their "Masters." Part of his plan includes using Ultron tech to help them achieve this goal. Crossbones kills Captain America toward the end of the film.

Ultron is unleashed and kills Zemo, taking over the Masters. Changes the plan to destroy the human race.

Iron Man
Hulk
Thor
Ant-Man
Wasp
Black Widow
Hawkeye
Falcon

Have to assemble to defeat Ultron and save the world. The glowing red power source used to create Ultron catches the eye of Thanos.