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Loki survived his fall and made a deal with the Chitauri, an alien race. Hand them the Tesseract and he gets an army to conquer Earth. He shows up at a S.H.I.E.L.D. base where the cube is being studied, uses a scepter to take over the minds of Agent Clint Barton, Hawkeye, and scientist Erik Selvig, and escapes with the Tesseract. The base collapses right after.
Nick Fury restarts the Avengers Initiative. Natasha Romanoff, the Black Widow, goes to Kolkata for Bruce Banner, who can track the cube's gamma signature. Fury shows a freshly thawed Steve Rogers what is at stake. Agent Phil Coulson brings the case to Tony Stark. In Stuttgart, Loki crashes a gala to steal iridium and makes the crowd kneel. Captain America and Iron Man take him on, and he gives up way too easily. On the plane home, Thor drops out of the sky, grabs his brother and tries to talk him down. After a brawl in the woods, they all bring Loki to the Helicarrier.
On board, nobody gets along. Tony and Steve snipe at each other, Banner trusts no one and Thor defends his brother. Digging around, Tony and Steve find out S.H.I.E.L.D. wanted to use the Tesseract to build weapons. Natasha figures out Loki wants to unleash the Hulk. Right then, a still-controlled Barton attacks the ship. Banner turns into the Hulk and chases Natasha, Thor fights him, and Tony and Steve fix an engine in midair. Loki escapes his cell, traps Thor in a cage that drops into the sky and kills Coulson. Natasha manages to snap Barton out of it.
Coulson's death changes the mood. Fury uses the agent's blood-stained baseball cards to rally the team. Tony realizes Loki wants an audience and will use Stark Tower in New York to open the portal. The Avengers arrive just as the Chitauri start pouring out of the sky. For the first time, Steve calls the shots for everyone. Banner rolls up on a motorcycle, says he is always angry and turns into the Hulk at will. In a moment every fan remembers, the Hulk slams Loki into the floor like a rag doll.
The council that runs S.H.I.E.L.D. fires a nuke at Manhattan. Tony catches the missile, flies it through the portal and sends it into the mothership. The blast shuts down every Chitauri on Earth. Natasha closes the portal with the scepter, and Tony falls back through, unconscious, until the Hulk catches him and a roar wakes him up. Thor takes Loki and the Tesseract to Asgard, and the heroes go their separate ways. In the credits scene, the Other tells his master, Thanos, that attacking Earth would be to court death. Thanos smiles.