The culmination of Marvel’s Phase One of movies, the Avengers brings together the characters introduced so far and pits them in a desperate struggle to save the Earth.
In a distant galaxy a mysterious stranger reports to his hidden master, announcing that their new ally, the fallen god Loki, has been given a mystic staff to control his enemies and command of their armies. His mission is to retrieve the Tesseract from Earth, and in return he will gain dominion over the planet, while the alien’s master will use its power to rule the universe.
On Earth, in a secret SHIELD facility where Project Pegasus has been experimenting on the Tesseract under supervision of Doctor Selvig, the scientists and agents notice the cube acting strangely. They worry that it will create an energy explosion that may destroy an unknowable area and begin to evacuate as Nick Fury and Maria Hill rush to the scene, meeting up with Selvig and Hawkeye. Instead of an explosion the cube creates a portal, which delivers Loki onto the scene. A fight ensues, but Loki is easily able to overcome the agents on the scene, and he uses his new staff to mind control Selvig and Hawkeye. He takes the cube and his new henchmen and races away from the base, leaving Fury for dead, with Agent Hill hot on his tail. The portal that delivered Loki becomes unstable and destroys the base, knocking Maria off of their trail. Fury manages to recover enough to give chase as well, but Loki shoots down his helicopter and escapes.
Given the threat the Tesseract could pose in the wrong hands, Fury decides that it is time to enact the Avengers initiative. He calls on Black Widow to collect Bruce Banner from his hiding place across the globe, and sends Agent Coulson to recruit Iron Man for research while he goes to retrieve Captain America. Cap and Banner are brought aboard the SHIELD helicarrier and begin to search for the cube using Banner’s expertise with gamma radiation.
Meanwhile, Loki is swinging his plan into action, attempting to build a machine that will allow the Tesseract to create a stable portal to transport his Chitari army to Earth. To make it he needs a rare element, and Hawkeye uses his skills to put together a plan to steal some in Germany. Loki goes to create a distraction and steal an eyeball to get Hawkeye past security, but his face pops up on a SHIELD facial recognition scan and Cap is quickly deployed to the scene to face off against him. He is soon joined by Iron Man and the two manage to capture Loki.
On the way back to the helicarrier their quinjet is attacked by Thor, who has managed to be transported back to Earth once they saw that Loki was present. Cap and Iron Man engage him, not knowing his intentions and not willing to lose control of Loki without knowing the location of the Tesseract, and the three fight themselves to a stand still before agreeing to return together with their prisoner.
Back aboard the helicarrier the group continue to search for the Tesseract, and to attempt to interrogate Loki, now locked in a prison intended an an emergency measure for the Hulk. Iron Man and Banner begin to suspect that SHIELD is holding something back about their work with the cube, and they begin to convince Cap of this as well, so as Iron Man attempts to hack their computers Cap begins to search the ship for himself. When he finds prototype weapons similar to the ones Hydra used during the second world war he returns to Stark and Banner. Widow discovers that Loki intended to get himself captured, and learns that his plan involves releasing the Hulk on the ship.
Banner, Stark, Cap, Fury, Widow, and Thor all converge in Banner’s lab and begin to argue about everything. Cap and Stark confront Fury about the Tesseract weapons and he admits that while its main use was going to be unlimited energy they were also developing the weapons to fight off extraterrestrial threats like Thor and Loki. Banner questions the real reason he is on the ship, and if they really intended to use the Hulk as a weapon. Loki’s staff seems to fuel their antagonism, and Banner barely manages to not change when the ship is suddenly attacked by Hawkeye, causing a massive explosion that takes out one of the main engines.
With the engine down and the mind controlled SHIELD agents attempting to take control of the ship, including a very determined Hawkeye, Iron Man and Cap rush into action to try to get the engine back online before the ship crashes. Black Widow finds herself face to face with the Hulk, racing through the lower decks to get away before Thor catches up to them and takes on the Hulk himself. Hawkeye manages to shut down a second engine just as Iron Man kickstarts the damaged one with some assistance from Captain America, and Hulk ends up leaping onto an attacking plane and getting thrown off onto the ground. Thor rushes to Loki’s prison only to find that he has already escaped, and he becomes trapped in the prison himself. Loki goes to drop the prison from the ship, but is interrupted by Coulson. He kills Coulson as Thor watches, and then dumps him. Thor is nearly killed in the fall, but manages to break his way out just before the cage hits the ground. Hawkeye runs into Black Widow and attempts to fight, but she manages to knock him out, restoring his free will in the process.
Beaten and bruised, Fury uses Agent Coulson’s death to inspire Cap and Iron Man to recommit to the team and the mission to retrieve the Tesseract. While Hawkeye doesn’t know where he is doing it, he does know that Loki is building a machine to make a portal, and that he’ll need a large power source. Stark comes to the realization that the perfect place would be his newly built Stark tower in New York, so the remaining team members rush into action to attempt to stop Loki’s plan from coming to fruition.
Stark flies into the tower and discovers Loki, taunting him while buying time for his new suit to be ready for action and barely making it. Thor arrives and challenges Loki, but they find that the portal machine is protected by an energy shield, and the two are too late to stop him from opening a portal for the alien army to pour through. Loki shoots down the ship carrying Captain America, Hawkeye, and Black Widow and they begin to engage the aliens on the street, along with Thor and Iron Man. Banner arrives during the fight and allows himself to transform into the Hulk again to help fight off the invaders.
The fight is hard and the heroes are barely holding in. They realize that they are not going to be able to win unless they find a way to close the portal, so they work on containment, joining together as a team, while Widow works her way back to the top of Stark Tower. Hulk beats the living tar out of Loki before moving on to more aliens, and Selvig explains to Widow that even under mind control he built a back door into the device, she just needs Loki’s staff, left behind after the Hulk’s attack, to pierce the energy shield and shut it down.
Unfortunately the team has a new problem. The council that commands SHIELD has decided that Fury’s Avengers plan isn’t cutting it, and they are determined to stop the alien threat by releasing a nuclear weapon to blow up Manhattan. He is unable to stop it from launching and calls on Iron Man for help. He is able to intercept the missile and carry it through the portal into the depths of distant alien space, directing the missile at an alien command ship. When the nuke blows the remaining aliens on the surface all shut down and die, and Widow is ordered to close the portal. To their surprise an unpowered Iron Man is able to fall back through the portal at the last moment, and is pulled from the air by the Hulk. The heroes regather themselves, having saved the city and once again capture Loki.
With the world saved the heroes each go their own way. Thor returns to Asgard with Loki and the Tesseract, and Fury assures the council that when the world needs them, the Avengers will assemble again.
TLDR: The Avengers come together to stop a threat from Loki, who is attempting to steal the Tesseract and take over the world with an alien army.