About a year after Captain America was defrosted, renegade Asgardian Loki appears on Earth and steals the Tesseract, a powerful device that can open gates to other parts of the universe which S.H.I.E.L.D. had been experimenting on since WWII. Loki also uses his staff to brainwash Erik Selving and Clint Barton a.k.a. Hawkeye into serving him.
S.H.I.E.L.D. director Nick Fury determines that in order to recover the Tesseract they have to resume the Avengers program, which had been suspended due to not finding enough candidates. Fury sends Coulson to go after Tony Stark and Natasha Romanoff a.k.a. Black Widow to recruit Bruce Banner, who's been hiding at India ever since his incident at Harlem. Fury himself goes to recruit Steve Rogers.
Bruce Banner is reluctant to trust S.H.I.E.L.D. and suspects they want to capture the Hulk but eventually agrees. Stark rejects the offer to join the team at first but agrees when he notices the Tesseract which was mentioned on his father's old research notes. Steve accepts Fury's offer to return to the battlefield but scolds S.H.I.E.L.D. for messing with the Tesseract as he witnessed what HYDRA did with it during WWII. Banner starts working on tracking the Tesseract while Captain America goes to Germany to capture Loki, which he succeeds at with Iron Man's help. On their way back to S.H.I.E.L.D.'s helicarrier their jet is attacked by Thor, who is still mad at Loki. Thor kidnaps Loki and tries to convince him to return home but Loki refuses. After a brief fight with Iron Man, Thor agrees to help them find the Tesseract, though he makes it clear that he intends to bring it back to Asgard as well as Loki so he can be properly incarcerated there.
At the Helicarrier Thor explains that Loki made a deal with the Chitauri alien race; they'll conquer Earth for him in exchange for the Tesseract. Stark also explains that Loki's using Selvig to build a device to activate the Tesseract so it opens a gate for the Chitauri army, then he goes to help with Banner's research. Captain America suggests they take a look at Loki's staff because it reminded him of HYDRA'S Tesseract fueled weapons. Fury interrogates Loki directly and quickly figures out that Loki let himself get caught on purpose. While working, Stark tries to convince Banner to stop fearing the Hulk and even tries to summon it. Captain America scolds Tony, only for Tony to reveal he's been hacking S.H.I.E.L.D. to get every dirty secret they've been hiding ever since he came to the helicarrier.
Tony and Banner suspect that there's something fishy going on with S.H.I.E.L.D.'s claims about using the Tesseract to make a clean energy source, as Stark has been working on the same project for over a year yet they haven't consulted for his help. Steve leaves enraged and decides to investigate on his own. Black Widow figures out that Loki plans to unleash the Hulk and wreck havoc. Before she can warn them however Fury confronts Stark about the hacking. Steve arrives and shows them Tesseract powered weapons. Banner finds out S.H.I.E.L.D. wants the Tesseract to build an arsenal of mass destruction weapons. Thor mocks humans for using the Tesseract like that, only for Fury to point out they're doing that because of his arrival on Earth last year. That incident made Earth realize that not only are there threats from other worlds, Earth is completely underpowered and defenseless against them.
Everyone continues to bicker until Loki's forces attack the helicarrier. During the attack, Banner Hulks out and jumps off the ship, Loki kills Coulson and Thor is ejected from the ship, but Hawkeye is freed from Loki's control. Fury takes advantage of Coulson's death to encourage the team to work together to avenge him. He also admits that he never wanted to follow the plan to build an arsenal but was forced to do so by the World Security Council, and that he geniunely believes the Avengers would be a better alternative to protect Earth.
Stark finally realizes that Loki will use Stark Tower to open the Chitauri gate to make it more flashy and flies there. Upon arriving he has a brief talk with Loki before the portal opens and the Chitauri invasion begins. Thor arrives and battles Loki. After Loki escapes Thor knocks out Selvig and tries to shut down the Tesseract machine generating the portal but fails. Captain America, Black Widow, and Hawkeye arrive too and begin to fight the invaders but they find themselves outmatched and Banner is nowhere to be found. A giant Leviathan monster comes from the gate which Iron Man distracts. Banner finally arrives, Hulks out, and kills the Leviathan with a single punch. Now with Hulk joining, the Avengers finally assemble and manage to work as a team.
The Avengers cripple as many Chitauri as possible, but they won't stop coming down and eventually the team starts getting exhausted. Black Widow goes to Stark Tower to try shut down the portal again while Hulk pummels Loki into submission. Meanwhile the World Security Council decides to send a nuclear missile to the city. Fury tries to stop the launch but the jet escapes so he calls Iron Man to warn him about the missile. Selvig, now free from the brainwash as well, tells Black Widow that Loki's scepter can turn off the portal. Before she can however, Iron Man takes the missile and sends it into the portal to destroy the Chitauri mothership which kills all the troops back at New York. Stark is knocked unconcious and falls down, seemingly to his death, but Hulk recues him. The invasion is over, the Avengers have won, and Loki is arrested.
The tired out Avengers go eat Shwarma in awkward silence.
Nick Fury decides to let Thor take the Tesseract and Loki back to Asgard and allows the Avengers go their own separate ways. The World Council scolds Fury for that decision and worries out the Avengers are a dangerous destructive force. Fury simply responds by pointing out that now other worlds know that Earth must not be underestimated and promises the Avengers will reunite when we need them again.
In the Post-credits scene Thanos is told about the Avengers' existence.
Why it Rocks
- First time Marvel's characters join in a major crossover movie.
- Samuel L.Jackson's portrayal of Nick Fury was fantastic.
- Every major Avenger already had an independent movie to introduce them and get to know them, so they all feel right in place on the team. Even Black Widow and Hawkeye don't feel out of place despite not having movies of their own.
- Mark Ruffalo managed to pull an amazing Hulk.
- Hulk vs Loki has become one of the funniest moments in the MCU.
- Epic fighting scenes and action.
- Triumphant sountracks to symbolize the Avengers assembling for the first time.
- Solidified the Marvel Cinematic Universe and set up major plots for future movies.
- Showed that Marvel can handle crossover movies.
- Tony and Steve's rivalry, which escalates into Captain America: Civil War starts developing here.
- Awesome tease of Thanos including his role as the big bad of the Marvel Cinematic Universe.
- It's impact, the success of this film would lead to other studios starting their own cinematic universes with interconnected plot threads, cameos, Easter eggs, as well as references to events of related movies.
- While Loki is still one of the few memorable MCU villains, the Chitauri on the other hand are just generic troop soldiers.
- Some scenes involving Captain America's character development were cut.