As the Avengers and their allies have continued to protect the world from threats too large for any one hero to handle, a new danger has emerged from the cosmic shadows: Thanos. A despot of intergalactic infamy, his goal is to collect all six Infinity Stones, artifacts of unimaginable power, and use them to inflict his twisted will on all of reality. Everything the Avengers have fought for has led up to this moment - the fate of Earth and existence itself has never been more uncertain.
银河系中的至尊霸主灭霸（乔什·布洛林饰）带着几个得力手下洗劫了全宇宙，只为了将所有的无限宝石镶嵌于金属手套上，这个手套可以将整个银河系毁灭于弹指间。为了拯救宇宙，托尼·斯塔克（小罗伯特·唐尼饰）和史蒂夫·罗杰斯（克里斯·埃文斯饰）需要摒弃前嫌，重组复仇者联盟，并与蜘蛛侠（汤姆·赫兰德饰）、奇异博士（本尼迪克特·康伯巴奇饰）、银河护卫队、黑豹（查德维克·博斯曼饰）以及瓦坎达人民的力量一同作战。
该片是《复仇者联盟》系列电影的第三部，是漫威电影宇宙的第19部电影，该片与《雷神3：诸神黄昏》剧情连接，讲述了复仇者联盟和他们的超级英雄盟友们牺牲一切，阻止灭霸毁灭一半宇宙的故事。