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Thanos wants all six Infinity Stones so he can wipe out half of all life in the universe with a snap. He already has the Power Stone, taken from Xandar, when he attacks the ship carrying Asgard's refugees. He pulls the Space Stone out of the Tesseract, beats the Hulk and kills Loki and Heimdall. Before he dies, Heimdall sends the Hulk to Earth through the Bifrost.
Banner crashes into the New York Sanctum and warns Doctor Strange and Wong. Thanos's children soon arrive for the Time Stone, which Strange carries. Ebony Maw takes Strange away on a ship, and Tony Stark and Peter Parker stow away after him. In Edinburgh, two more of Thanos's lieutenants attack Wanda and Vision, who has the Mind Stone in his forehead. Steve Rogers, Natasha and Sam rescue them, and the group takes Vision to Wakanda, where Shuri tries to remove the Stone without killing him.
In space, the Guardians of the Galaxy pick up Thor. He heads to Nidavellir with Rocket and Groot and forges the axe Stormbreaker, nearly dying in the process. Quill, Gamora, Drax and Mantis go to Knowhere, but Thanos already has the Reality Stone and takes Gamora. To save Nebula from torture, she reveals where the Soul Stone is. On Vormir, the Red Skull explains the price: losing someone you love. Thanos weeps and throws Gamora off the cliff.
Tony and Peter rescue Strange and land on Titan, Thanos's dead home world, where they run into Quill, Drax and Mantis. They come up with a plan to pull off the Gauntlet. Mantis puts Thanos under and they almost do it, but Quill learns Gamora is dead and loses it. Thanos wakes, beats them all and wounds Tony. Strange, who saw 14,000,605 possible futures and only one where they win, hands over the Time Stone to spare Tony's life.
In Wakanda, Thanos's army attacks. Thor arrives with Rocket and Groot and turns the fight, but then Thanos shows up. Wanda destroys the Mind Stone along with Vision, but Thanos uses the Time Stone to rewind a few seconds, rips the Stone out and kills Vision. Thor buries Stormbreaker in his chest, and Thanos tells him he should have gone for the head. Thanos snaps. Half of all life vanishes: Bucky, T'Challa, Groot, Wanda, Sam, Mantis, Drax, Quill, Strange and Peter, who dies in Tony's arms, all turn to dust. Thanos retreats to a quiet planet to watch the sunrise. In the post-credits scene, Nick Fury and Maria Hill fade away too, but Fury manages to send a signal on a pager. Who will answer is left open.