The Asgardian Loki encounters the Other, the leader of an extraterrestrial race known as the Chitauri. In exchange for retrieving the Tesseract, a powerful energy source of unknown potential, the Other promises Loki an army with which he can subjugate Earth. Nick Fury, director of the espionage agency S.H.I.E.L.D., and his lieutenant Agent Maria Hill arrive at a remote research facility during an evacuation, where physicist Dr. Erik Selvig is leading a research team experimenting on the Tesseract. Agent Phil Coulson says that the object has begun radiating an unusual form of energy. The Tesseract suddenly activates and opens a wormhole, allowing Loki to reach Earth. Loki takes the Tesseract and uses his scepter to enslave Selvig and a few other agents, including Clint Barton, to aid him in his getaway.
In response to the attack, Fury reactivates the Avengers Initiative. Agent Natasha Romanoff sends to Kolkata to recruit Dr. Bruce Banner to trace the Tesseract through its gamma radiation emissions. Coulson visits Tony Stark to have him check Selvig’s research, and Fury approaches Steve Rogers with an assignment to retrieve the Tesseract.
In Stuttgart, Barton steals iridium needed to stabilize the Tesseract’s power while Loki causes a distraction, leading to a brief confrontation with Rogers, Stark, and Romanoff that ends with Loki’s surrender. While Loki gets escorted to S.H.I.E.L.D., Thor, his adoptive brother, arrives and frees him, hoping to convince him to abandon his plan and return to Asgard. After a confrontation with Stark and Rogers, Thor agrees to take Loki to S.H.I.E.L.D.’s flying aircraft carrier, the Helicarrier. Upon arrival, Loki gets imprisoned while Banner and Stark attempt to locate the Tesseract.
The Avengers become divided, both over how to approach Loki and the revelation that S.H.I.E.L.D. plans to harness the Tesseract to develop weapons as a deterrent against hostile extraterrestrials. As the group argues, Barton and Loki’s other possessed agents attack the Helicarrier, disabling one of its engines in flight and causing Banner to transform into the Hulk. Stark and Rogers work to restart the damaged engine, and Thor attempts to stop the Hulk’s rampage. Romanoff reluctantly fights Barton, and knocks him unconscious, breaking Loki’s mind control. Loki escapes after killing Coulson and ejecting Thor from the airship, while the Hulk falls to the ground after attacking a fighter jet. Fury uses Coulson’s death to motivate the Avengers into working as a team. Stark and Rogers realize that for Loki, simply defeating them is not enough; he needs to overpower them publicly to confirm himself as ruler of Earth. Loki uses the Tesseract, and a wormhole generator Selvig built, to open a wormhole above Stark Tower to the Chitauri fleet in space, launching his invasion.
Rogers, Stark, Romanoff, Barton, and Thor rally in defense of New York City. Banner arrives and transforms into the Hulk, and together the Avengers battle the Chitauri while evacuating civilians. The Hulk beats Loki into submission. Romanoff makes her way to the generator, where Selvig, freed from Loki’s mind control, reveals that Loki’s scepter can shut down the generator. Meanwhile, Fury’s superiors from the World Security Council attempt to end the invasion by launching a nuclear missile at Midtown Manhattan. Stark intercepts the missile and takes it through the wormhole toward the Chitauri fleet. The missile detonates, destroying the Chitauri mothership and disabling their forces on Earth. Stark’s suit loses power, and he falls back through the wormhole just as Romanoff closes it. Stark goes into freefall, but the Hulk saves him from crashing into the ground. In the aftermath, Thor returns Loki and the Tesseract to Asgard, while Fury expresses confidence that the Avengers will return if needed.
In a mid-credits scene, the Other confers with his master about the failed attack on Earth.
The first in the Avengers movies where all these awesome superheroes are working as a team, which was the point of S.H.I.E.L.D. Working together Thor, Hulk, Iron Man, Captain America and Black Widow and later with Clint Barton (Hawkeye) defeat Loki and the Chitauri. Now this movie does set up a bit for the second Avengers, Avengers: Age of Ultron as well as Captain America: Civil War, which are further down in the list as you can see in the viewing order below. This movie is full of action, hints to back stories already released and family friendly.