Nick Fury of S.H.I.E.L.D. assembles a team of superhumans to save the planet from Loki and his army.
Nick Fury, the director of the espionage agency known as S.H.I.E.L.D. arrives at Project P.E.G.A.S.U.S., a remote research facility, during an evacuation. His second-in-command, Maria Hill, explains that the Tesseract, an energy source of unknown potential, has activated and opened a portal through space, from which the exiled Asgardian prince Loki, steps through. Loki takes the Tesseract, and uses a sceptre to control the minds of several S.H.I.E.L.D. personnel including Agent Clint Barton, and physicist consultant Dr. Erik Selvig in order to aid in his getaway.
In response, Fury reactivates the Avengers Initiative. Agent Natasha Romanoff recruits Dr. Bruce Banner in India, while Agent Phil Coulson visits Tony Stark and requests that he reviews Selvig's research. Fury himself approaches Steve Rogers with an assignment to retrieve the Tesseract from Loki; Rogers is familiar with the Tesseract as it played a vital role in his World War II exploits. Romanoff, Coulson, Rogers, and Banner meet on the S.H.I.E.L.D. Helicarrier before it goes airborne. Dr. Banner is escorted to his lab after a brief meeting with Fury; Banner has been consulted to trace the gamma signature that the Tesseract radiates. After spending some time tracking the whereabouts of the Tesseract on the Helicarrier, a man of Loki's description is located in Europe.
Iron Man, Captain America, and Romanoff travel to Stuttgart, Germany to apprehend Loki, who, with the aid of a mind-controlled Barton, is stealing iridium needed to stabilize the Tesseract and demanding worship. After a battle with the heroes, Loki surrenders and is being returned to the Helicarrier in the Quinjet. Thor, Loki's adoptive brother and the Asgardian god of thunder, attempts to free Loki and reason with him. After a confrontation with Iron Man and Captain America, Thor accompanies them to imprison Loki on the Helicarrier. After Loki is put into the cell originally intended for the Hulk, Thor reveals to the team that Loki is allied with the Chitauri, a powerful and technologically-advanced extraterrestrial race unknown to both Earth and Asgard. Stark has J.A.R.V.I.S. hack into the S.H.I.E.L.D. database while he works with Dr. Banner. Rogers is less than thrilled at Stark's behaviour and tells the two scientists to resume their work and investigates Phase Two, S. H. I. E. L. D.'s top secret weapons program. While all of this is going on, Romanoff speaks one-on-one with Loki, and eventually discovers that he plans on using the Hulk as a means of dismembering the team.
After all of these things come to light, the Avengers are divided over how to deal with Loki and the revelation that S.H.I.E.L.D. plan to harness the Tesseract’s power to develop weapons. Fury admits that the events in New Mexico a year before made S.H.I.E.L.D. aware of other extra-terrestrial races that may threaten Earth. Weapons developed from the Tesseract would form a means of deterrence. As the group argues, Barton and Loki’s possessed agents attack the Helicarrier, disabling its engines. As Iron Man and Captain America attempt to restart the damaged engines, Banner transforms into the Hulk, despite Romanoff's efforts to calm him, and runs amok inside the ship, soon battling Thor. During a fight with Barton, Romanoff discovers that knocking him unconscious breaks Loki's mind control. Loki escapes his cell with the help of a possessed agent and traps Thor in the cell. Coulson confronts Loki in an attempt to rescue Thor, and Loki then stabs Coulson through his back with his staff before ejecting Thor from the ship. Fury confronts the expiring Coulson, who hopes his death can be used to motivate the Avengers into working as a team.
Stark and Rogers work out that Loki will orchestrate the alien invasion at Stark's New York-based home, Stark Tower, and that defeating them will not be enough for Loki; he needs to overpower them in a public way so as to validate himself as ruler of Earth. Using a Tesseract-powered interdimensional generator Selvig has built, Loki opens a portal to the Chitauri fleet over Manhattan, summoning an invasion. The Avengers rally in defence of New York, but quickly conclude they will be overwhelmed as wave after wave of Chitauri and Leviathans descend. The Avengers keep their ground to hold off the invasion and evacuate civilians while the Hulk beats Loki into submission. Romanoff makes her way to Selvig's device, where Selvig, freed of Loki's control, reveals that Loki's staff can be used to close the portal.
Meanwhile, Fury's superiors attempt to end the invasion by launching a nuclear missile at Manhattan. Despite Agent Hill and Fury's orders to not fire, a rogue jet launches the missile at Manhattan. Iron Man intercepts it and takes it through the portal toward the Chitauri fleet before running out of power and plummeting back to Earth. The Hulk catches him as he falls. The Avengers then apprehend Loki, who is still weak from his encounter with the Hulk. News channels confirm the legitimacy of the extra-terrestrial attack and some Americans regard the Avengers with praise and thanks while others call for their arrests. Thor escorts Loki and the Tesseract back to Asgard to take responsibility for his crimes. Fury notes that the Avengers will go their separate ways until such time as a new world-threatening menace emerges.
In the first post-credits scene, The Other tells his master that humans are not the "cowering wretches" they were promised and that attacking Earth again "would be to court Death". His master, Thanos turns and smiles.
In a second post-credits scene, the Avengers — gathered at a shawarma restaurant and looking a bit worse for the wear — eat in silence.
- Robert Downey Jr. - Tony Stark
- Chris Evans - Steve Rogers
- Mark Ruffalo - Bruce Banner
- Chris Hemsworth - Thor Odinson
- Scarlett Johansson - Natasha Romanoff
- Jeremy Renner - Clint Barton
- Tom Hiddleston - Loki Laufeyson
- Clark Gregg - Phil Coulson
- Cobie Smulders - Maria Hill
- Stellan Skaragard - Erik Selvig
- Samuel L. Jackson - Nick Fury