The Asgardian Loki (Tom Hiddleston) finds himself in contact with the Other, the commander of an alien race known as the Chitauri. In exchange for retrieving the Tesseract, an immense source of untold energy, Loki is promised an army to help him conquer Earth. This Tesseract, which was once lost when Captain America brought Hydra to its knees, has formidable potential as a sustainable energy source.
Years prior, Howard Stark (Tony Stark's father) discovered the Tesseract while searching for Captain America on the ocean floor. When Nick Fury (Samuel L. Jackson), director of S.H.I.E.L.D., and his right-hand agent, Maria Hill, rush to a research facility during an evacuation, they find physicist Dr. Erik Selvig operating an experiment involving the Tesseract. Agent Phil Coulson informs them that the object has begun emitting a strange energy. Suddenly, the Tesseract activates and opens a portal through which Loki arrives on Earth. Using his scepter, he enslaves Selvig and several agents, including Clint Barton (Jeremy Renner), manipulating them into assisting with his escape. Loki boldly declares his plan to subjugate humanity to Fury.
In a desperate move, Fury sets the facility to self-destruct, but both Barton and Selvig manage to relay crucial information to Loki before fleeing with him. The facility is obliterated.
In response to Loki’s threat, Fury reactivates the Avengers Initiative. The mission begins with Agent Natasha Romanoff (Scarlett Johansson) sent to Calcutta to enlist Dr. Bruce Banner (Mark Ruffalo), who can trace the Tesseract via its gamma radiation. Additionally, Coulson approaches Tony Stark (Robert Downey Jr.) to analyze Selvig's data, while Fury recruits Steve Rogers (Chris Evans) to retrieve the Tesseract. It’s revealed that Banner's experiments to recreate the original formula that created Captain America resulted in the creation of the Hulk.
Loki, believing himself the rightful heir of Asgard, plans to conquer Earth, claiming it was wrongfully taken and given to his brother Thor. The Other further reveals their intention to dominate other worlds as uncovered by the Tesseract. Loki's eventual betrayal leads to Jonathan Barton stealing iridium necessary for stabilizing the Tesseract while Loki preoccupies the heroes in Stuttgart, resulting in a confrontation where Loki ultimately surrenders.
During the escort back to S.H.I.E.L.D., Thor arrives, freeing Loki in hopes of persuading him to return to Asgard. Despite Thor's insistence that Earth is under his protection, Loki has hidden the Tesseract. After a dramatic disagreement with Stark and Rogers, Thor decides to transport Loki to S.H.I.E.L.D.'s aerial carrier, the Heli-carrier, where Loki is detained while Banner and Stark try to pinpoint the Tesseract's location.
Onboard, Stark reveals Loki's cunning plan involving Barton stealing iridium to keep the portal open, ensuring a gateway for the Chitauri army. They discover that Loki intends to utilize Stark's Arc Reactor as a power source. Tensions rise within the group as debates arise over how to confront Loki, especially considering S.H.I.E.L.D.'s secret agenda to weaponize the Tesseract against potential extraterrestrial threats. This discord leads to an assault on the Heli-carrier by Barton and other agents, resulting in Banner transforming into the Hulk.
As Stark and Rogers scramble to fix the engines, Thor attempts to contain the Hulk's fury while Romanoff subdues Barton, breaking Loki's control over him. This revelation shows that Loki had orchestrated everything to trap the Avengers and exploit the ensuing chaos for his own escape. With the destruction of Coulson, Fury galvanizes the Avengers, emphasizing their unity as the only means to thwart Loki's ambitions.
Loki realizes that he must publicly defeat the Avengers to legitimize his claim to Earth. Consequently, he activates the Tesseract alongside the iridium device to unleash the Chitauri forces from above Stark Tower. The Avengers brace themselves to defend New York City against the impending attack. Amidst the chaos, Rogers, Stark, and Thor collaborate to assist civilians, while Banner returns as the Hulk to take down Loki.
As Romanoff rushes to the portal, Selvig, freed from Loki's mind control, discloses that they can use Loki’s scepter to close the portal. Meanwhile, Fury's higher-ups decide to launch a nuclear missile toward Manhattan to quell the invasion. In a daring maneuver, Stark intercepts the missile and directs it through the portal, targeting the Chitauri fleet. The detonation obliterates their leading ship, severely crippling their forces on Earth. Stark’s Iron Man suit loses power as he returns through the portal, only to be rescued by the Hulk, while Romanoff shuts down the portal to cease further attacks.
In the aftermath, Thor returns both Loki and the Tesseract to Asgard, and Fury assures that the Avengers will be called upon again if needed. In a light-hearted moment in the first post-credits scene, the Other discusses humanity's resistance with his master, while the second scene features the Avengers sharing a quiet meal in a Shawarma restaurant, reflecting on the chaos they’ve just endured.
