When laying out plans for the Marvel Cinematic Universe, Kevin Feige was always clear that he saw the franchise as a world of individual movies starring individual superheroes having individual Story Arcs, followed by ensemble movies focusing on a Super Team that would craft a larger Myth Arc out of the stories and characters introduced by prior installments. As such, the final movie of the MCU's "Phase One" was one such larger movie. It was very well-received and was, at the time, the third-highest-grossing movie in history. (It has since been surpassed by several other films, including two of its own sequels.)
We begin with Loki (Tom Hiddleston) meeting with a representative (The Cameo of Alexis Denisof) of an alien race called the Chitauri. The representative will give Loki an army with which to invade Earth, but in exchange he (the representative) wants "the Tesseract," a glowing blue cube last seen in the custody of S.H.I.E.L.D. Speaking of which, S.H.I.E.L.D.'s leader, Nick Fury (Samuel L. Jackson), arrives at a remote facility where Dr. Erik Selvig (Stellan Skarsgård) is studying the Tesseract. It opens a portal through which Loki steps. Loki, using a scepter which has Mind Control powers, subdues many of the occupants of the facility, including Selvig and Clint "Hawkeye" Barton (Jeremy Renner). Fury barely escapes with the help of his aide and Number Two, Maria Hill (Cobie Smulders), while Loki begins to build his army via hypnosis.
Nick Fury activates the "Avengers Initiative." He sends Natasha "Black Widow" Romanoff (Scarlett Johansson) to recruit Bruce Banner (meant to be the character played by Edward Norton, but recast with Mark Ruffalo), who has been hiding out since the events of his movie, so that Banner can track the gamma radiation from the Tesseract. Steve "Captain America" Rogers (Chris Evans) is also brought on board, and Agent Phil Coulson (Clark Gregg) ropes in Tony "Iron Man" Stark (Robert Downey Jr.).
Stark, Banner, Romanoff and Rogers arrive at Stuttgart, where Hawkeye is helping Loki steal iridium to stabilize the Tesseract. Captain America, Black Widow and Iron Man succeed at apprehending Loki, but he is unexpectedly freed by his half-brother, Thor (Chris Hemsworth), who is trying to get Loki to make a Heel–Face Turn. After a short "Let's You and Him Fight" interlude, the Avengers, now augmented by Thor, imprison Loki on a S.H.I.E.L.D. Airborne Aircraft Carrier.
Fury argues with his superheroes over what to do with Loki, as well as his intent to use the Tesseract — which houses a Green Rock of unfathomable power; you know, almost an infinite stone — to develop weapons for defense against extraterrestrial threats (like Thor and Loki). Meanwhile, the heroes argue amongst themselves, particularly the pragmatic Tony and the more Cape-minded Steve, who accuses Tony of being unwilling to make "the sacrifice play." They are disrupted when Loki's mooks, led by Barton, attack the Helicarrier. While Stark and Rogers work to keep the ship in the sky, Banner is driven to Hulk out by anxiety and eventually jumps overboard. (He survives.) Meanwhile, Romanoff is able to break the mind-control on Barton, happily rescuing her old friend. But Loki, in the chaos, is able to escape... and kills Coulson in the process.
Fury uses Coulson's death to motivate the six superheroes into actually becoming a team; Tony inadvertently names them by declaring, "if we can't protect the Earth, you can be damn well sure we'll avenge it."
Loki uses the Tesseract to open a wormhole in New York City, right above Stark Tower, through which he begins to summon his Chitauri army. The Avengers race to the defense, and Hulk manages to beat Loki into submission — he may be invulnerable to damage, but he can still be tossed around. Romanoff gets to the wormhole generator and frees Selvig, but Selvig reveals that only Loki's staff can close the wormhole. Meanwhile, Fury is contacted by an Omniscient Council of Vagueness, who decide to end the whole problem by nuking NYC from orbit. Stark, hearing of this, proves Steve wrong by making "the sacrifice play": he intercepts the warhead and flies it up into the wormhole, where its destructive power wipes out the Chitauri mothership. Even better, the Chitauri are a Keystone Army, and with their mothership gone, they power down. All that's left is Tony, falling lifelessly down to the planet below. But the Hulk catches him, and Tony survives, while Romanoff uses Loki's scepter to close the wormhole.
Thor takes Loki and the Tesseract to Asgard, where Loki will face justice. The remaining five Avengers take up residence at Stark Tower, which was damaged in the fight: all the big neon consonants were knocked off, leaving only the big red A. And, during The Stinger, the Chitauri representative reports to his master — tall, craggy-faced, purple — about the failed invasion of Earth...
