自從上一回紐約遭到外星人入侵，人民得知有超級英雄的存在，從此生活的每一刻都變得膽顫心驚。不敢再冒險的復仇者聯盟首領東尼史塔克，決定重啟塵封已久的和平計劃，打造了一台叫做「奧創」的機器人，好可以分擔所有英雄的救援能力。

但就在這個時候，「奧創」竟然活生生轉變成極端偏激、具有自我思想的殺人機器，揚言要統治這個世界。毫無預警且感到不安的成員們，開始因為史塔克所製造的麻煩，而產生了分歧...。

When Tony Stark tries to jumpstart a dormant peacekeeping program, things go awry and Earth's Mightiest Heroes, including Iron Man, Captain America, Thor, The Incredible Hulk, Black Widow and Hawkeye, are put to the ultimate test as the fate of the planet hangs in the balance. As the villainous Ultron emerges, it is up to The Avengers to stop him from enacting his terrible plans, and soon uneasy alliances and unexpected action pave the way for a global adventure.