The Avengers

We open on a member of the alien race called Chitauri talking about the tesseract (an object of pure energy) that was lost on Earth during World War II, but recently found. He says that the humans don't understand its power, and that it can open a gateway between their world and ours. The alien says this with a hidden entity, the mastermind of the villainous plot. They assign their new ally Loki  (Thor's adopted brother who had been sucked into an abyss at the end of Thor) to obtain the tesseract and use its power to open a gateway between Chitauri space and Earth. In return, the Chitauri army will help Loki take over Earth.

We see Nick Fury, the head of the worldwide defense operation SHIELD is experimenting on the Tesseract as it possesses a potentially unlimited source of clean energy. The tesseract then opens a small portal allowing Loki through. Loki easily takes over the guards, and then takes control of the minds of SHIELD super hero/agent Clint Barton/Hawkeye (Jeremy Renner) and SHIELD astrophysicist Selvig (Stellan Skarsgard). He steals the tesseract, despite the attempts of Nick Fury and agent Maria Hill (Cobie Smulders) to stop them.

Fury decides to activate the Avengers protocol. The team is eventually collected, consisting of SHIELD spy Natasha Romanova/Black Widow, industrialist/inventor Tony Stark/Iron-Man, super-soldier Steve Rodgers/Captain America, and scientist Bruce Banner/Hulk   Banner is brought on only to serve as a gamma radiation consultant because as the Hulk he is too volatile and dangerous to be on the team. The team captures Loki in Germany, not knowing that Hawkeye has managed to obtain another important piece to enact the Tesseract portal while Loki was causing a distraction.

Asgardian god and Loki's brother Thor is transported back to Earth by their father Odin (Loki is Odin's adopted son) to take Loki and the tesseract back to Asgard. A fight ensues between Captain America, Iron-Man and Thor over Loki's custody, but then agree to keep him incarcerated with SHIELD until the tesseract is recovered. Thor allies himself with the team.  Iron-Man, Captain America, and Banner eventually discover that SHIELD had used the tesseract to create a more powerful "nuclear" bomb to defend the Earth from extraterrestrial forces such as the Asgardians. Black Widow interrogates Loki and tricks him into revealing that he is manipulating Banner to release the Hulk (hoping the Hulk will either kill the Avengers or cause them to come apart). Before they can evacuate Banner, Hawkeye has a sneak attack on the Hellicarrier - SHIELD's flying fortress, damaging one of the helicarrier's engines. During the struggle, the Hulk is unleashed and almost kills Black Widow before being fought and almost defeated by Thor; Loki is released and beats and humiliates Thor; Loki murders Agent Coulson as Coulson attempts to save Thor and defeat Loki; Black Widow fights Hawkeye and knocks him out freeing him of Loki's mind control; and Iron-Man and Captain America manage to repair the hellicarrier's engine before it crashes.

Having regained his mind, Hawkeye agrees to become the final member of the Avengers to take revenge on Loki for manipulating him into committing crimes. The Avengers finally unite around Agent Coulson's heroic death and discover that Loki is looking to use Stark Towers to power the tesseract portal. The Avengers arrive too late to stop Loki from opening the portal, but Iron-Man vows to avenge the Earth if it can't be saved.

A large fight starts between the aliens, led by Loki, and the Avengers, led by Captain America. Banner manages to rejoin them and reveals that, when focused, he has general control of The Hulk. Banner becomes Hulk on the alien army. Thor fights with and tries to have Loki stand down to no avail. Meanwhile, the council orders Nick Fury to initiate Phase 2 - which is the plan to use the super atomic bomb to destroy the portal, but also New York at the same time.  Although both Fury and Hill refuse to obey these orders, agents working directly for the council manage to shoot the weapon at New York where the battle is taking place.

Eventually the Hulk beats down and defeats Loki. Iron-Man realizes that the bomb has been launched at NYC, and he intercepts it, flying it through the portal to Chitauri space before falling there. The bomb strikes the Chitauri space center, killing all of the Chitauri soldiers on Earth. Selvig, who has recovered from Loki's control since Loki's defeat, explains that he built a failsafe in the tesseract to shut the portal despite his being under mind control. Black Widow activates the failsafe and shuts the portal between Earth and the Chitauri planet. Iron-Man's seemingly lifeless body falls back through the portal to Earth just before the portal is closed. The heroes are about to mourn Iron-Man's death, when he miraculously recovers. The team reminds him that they still have to take Loki into custody, which is easy since he is still severely injured from his fight with the Hulk and his army defeated.

The majority of the world welcome the Avengers as heroes.  Nick Fury lets the team to seperate and pursue their own private lives, much to the displeasure of the council who now wants the Avengers fully under their control. Loki and the tesseract are handed over to Thor, who takes them back to Asgard. The others go their own way. We watch as Tony Stark makes plans for what will likely be Avengers tower, along with Pepper Pots who now wears an engagement ring. Nick Fury assures Maria Hill and the council that when they are needed, the Avengers will be back once again.