Avengers Assemble

"Avengers Assemble" is a crossover event, with episodes of the television series Thor, Iron Man, and Captain America on NBC, and is inspired by the 2002 comic miniseries The Ultimates. The crossover began on May 18, 2015 with Thor, continued on Iron Man on May 19, and concluded on Captain America on May 20. Each of the three episodes were titled "Avengers Assemble". In the crossover, Nick Fury of S.H.I.E.L.D. recruits Tony Stark, Steve Rogers, and Bruce Banner to join S.H.I.E.L.D. agents Natasha Romanoff and Clint Barton to stop an alien invasion, before being joined by Thor.

Development of a crossover between the three series began in May 2014, when Thor and Captain America were ordered to series by NBC, to join fellow superhero series Iron Man. By the following February the scripts for each episode were written, and were filmed in March 2015. The premise and episode titles were also announced in February. In "Avengers Assemble", the main cast members of each series appear in (at least) their own series; Jason O'Mara reprises his role as Bruce Banner from The Incredible Hulk in the entire crossover.

The three episodes of the crossover received positive reviews, with critics praising the interaction among the actors in each series and the visual effects surpassing standards set by network television. Ratings for the episodes were high, with each breaking records in viewership and the 18–49 rating. A second crossover, "Age of Ultron", occurred the following year.

Plot
The Asgardian Loki encounters the Other, the leader of an extraterrestrial race known as the Chitauri. In exchange for retrieving the Tesseract, a powerful energy source of unknown potential, the Other promises Loki an army with which he can subjugate Earth. Nick Fury, director of the espionage agency S.H.I.E.L.D., and his lieutenant Agent Maria Hill arrive at a remote research facility during an evacuation, where physicist Dr. Erik Selvig is leading a research team experimenting on the Tesseract. Agent Phil Coulson explains that the object has begun radiating an unusual form of energy. The Tesseract suddenly activates and opens a wormhole, allowing Loki to reach Earth. Loki takes the Tesseract and uses his scepter to enslave Selvig and a few other agents, including Clint Barton, to aid him in his getaway. In response to the attack, Fury reactivates the "Avengers Initiative". Agent Natasha Romanoff is sent to Kolkata to recruit Dr. Bruce Banner to trace the Tesseract through its gamma radiation emissions. Coulson visits Tony Stark to have him review Selvig's research, and Fury approaches Steve Rogers with an assignment to retrieve the Tesseract. In Stuttgart, Barton steals iridium needed to stabilize the Tesseract's power while Loki causes a distraction, leading to a brief confrontation with Rogers, Stark, and Romanoff that ends with Loki's surrender. While Loki is being escorted to S.H.I.E.L.D., Thor, his adoptive brother, arrives and frees him.

Thor hopes to convince Loki to abandon his plan and return to Asgard. After a confrontation with Stark and Rogers, Thor agrees to take Loki to S.H.I.E.L.D.'s flying aircraft carrier, the Helicarrier. Upon arrival, Loki is imprisoned while Banner and Stark attempt to locate the Tesseract. The Avengers become divided, both over how to approach Loki and the revelation that S.H.I.E.L.D. plans to harness the Tesseract to develop weapons as a deterrent against hostile extraterrestrials. As the group argues, Barton and Loki's other possessed agents attack the Helicarrier, disabling one of its engines in flight and causing Banner to transform into the Hulk. Stark and Rogers work to restart the damaged engine, and Thor attempts to stop the Hulk's rampage. Romanoff reluctantly fights Barton, and knocks him unconscious, breaking Loki's mind control. Loki escapes after killing Coulson and ejecting Thor from the airship, while the Hulk falls to the ground after attacking a S.H.I.E.L.D. fighter jet. Fury uses Coulson's death to motivate the Avengers into working as a team. Stark and Rogers realize that for Loki, simply defeating them will not be enough; he needs to overpower them publicly to validate himself as ruler of Earth. Loki uses the Tesseract, in conjunction with a device Selvig built, to open a wormhole above Stark Tower to the Chitauri fleet in space, launching his invasion.

Rogers, Stark, Romanoff, Barton, and Thor rally in defense of New York City, the wormhole's location. Banner arrives and transforms into the Hulk, and together the Avengers battle the Chitauri while evacuating civilians. The Hulk finds Loki and beats him into submission. Romanoff makes her way to the wormhole generator, where Selvig, freed from Loki's mind control, reveals that Loki's scepter can be used to shut down the generator. Meanwhile, Fury's superiors from the World Security Council attempt to end the invasion by launching a nuclear missile at Midtown Manhattan. Stark intercepts the missile and takes it through the wormhole toward the Chitauri fleet. The missile detonates, destroying the Chitauri mothership and disabling their forces on Earth. Stark's suit loses power, and he falls back through the wormhole just as Romanoff closes it. Stark goes into freefall, but the Hulk saves him from crashing into the ground. In the aftermath, Thor returns Loki and the Tesseract to Asgard, while Fury expresses confidence that the Avengers will return if and when they are needed.

In a mid-credits scene, the Other confers with his master, Thanos, about the failed attack on Earth

Development
Kevin Feige had originally envisioned several films based on the Marvel Comics heroes, leading to crossover films every few years, before leading to one big climactic fight. However, after being turned down, Feige switched his vision to television series, knowing that the series would likely be a very different approach to network television, due to many of the heroes having their film trilogies turned into three-season series, different than regular network series, which are designed to go on for several years. However, Feige managed to develop both Iron Man and The Incredible Hulk as test runs before other series could be put into development depending on the success of those series. Though The Incredible Hulk did not do well, having its episode order decreased from thirteen to ten episodes, before being cancelled as a whole. Iron Man was a bigger success, being the third most-watched NBC series of the 2013–14 season, behind only The Voice and Chicago Fire, and the thirty-seventh overall. With the success of Iron Man, NBC put Thor and Captain America into development, and both were picked up to series in May 2014.

Despite NBC's weariness to continue the series due to the main successes coming from DVR viewing rather than live viewing, they allowed Feige to develop a crossover event that would encompass the finales of each of the series' current seasons' finales. In February 2015, the scripts for each of the three episodes were written and the Chitauri was revealed to be the antagonists of the event, led by Loki. It was also revealed that the event would take inspiration from the Ultimates comic series, and that Jason O'Mara would return as Bruce Banner/Hulk.

Writing
Joss Whedon was revealed to be writing and directing all three episodes, with the Thor episode overseen by showrunners Zack Stenz and Ashley Miller, Iron Man by showrunner Jon Favreau, and Captain America by showrunners Christopher Markus and Stephen McFeely.

Filming
The episodes were filmed concurrently by Joss Whedon across March 20 to April 8, 2015. It was filmed in New York City.

Marketing
The crossover was heavily promoted by NBC in order to attract viewers. The event's trailer was released on April 29, 2015.

Critical response
The crossover as a whole received praise for Whedon's direction and screenplay, visual effects, action sequences, acting, and musical score. On an episode-by-episode basis, IGN gave the crossover an overall "Amazing" score of 9/10, with Thor receiving an 8.1, Iron Man receiving a 9.7, and Captain America receiving a 9.2.