Avengers: Age of Ultron builds upon the events of the Marvel Cinematic Universe that took place after Earth's Mightiest Heroes first assembled for The Avengers in 2012, but there are other events that occurred in the background since then that most moviegoers will not have seen or read.

The sequel throws viewers right into the action with the team already assembled on a mission against a Hydra base. Captain America, Iron Man, Hulk, Thor, Black Widow and Hawkeye are together again and they're on a mission to take down Hydra's current top official, Baron von Strucker, and retrieve the dangerous and mysterious scepter left behind from Loki's attack on Earth.

How the Avengers discovered the location of this base and the experimental work being conducted by Strucker here isn't explored in the limited run time of the film but it's something Marvel has been building towards for quite sometime.

So, if you're a fan of the films and want some background information of the events leading up to the Age of Ultron, or maybe you missed a few releases along the way since this is the 11th film in the franchise, we're here to help.

Where Is S.H.I.E.L.D.?

Samuel L. Jackson's Nick Fury Marvel Studios Contract

It was S.H.I.E.L.D. Director Nick Fury's idea to spearhead the Avengers Initiative - a plan he believed in strongly that would gather the rare extraordinary people of the world in a time of need to protect against the most dangerous of threats. The secretive organization (full name: Strategic Homeland Intervention, Enforcement and Logistics Division) is a high-tech, elite international spy agency and it remained in charge of worldwide security after The Avengers first assembled to protect New York from Loki and his Chitauri alien invaders and then went their separate ways.

S.H.I.E.L.D. was torn apart from the inside months later in Captain America: The Winter Soldier when it was revealed that Hydra (Red Skull's faction from World War II) was still active and had spent decades infiltrating S.H.I.E.L.D. and various elements of government. With S.H.I.E.L.D.'s leadership, the world security council, now out of the picture and director Nick Fury believed dead, the organization was no more the force it once was.

Instead, there only remained small teams of trusted operatives lead by Agent Coulson (Clark Gregg) who was appointed the new director by Fury (who's still alive). This storyline can only be seen in in Marvel's Agents of S.H.I.E.L.D. TV series on ABC.

When Did The Avengers Re-Assemble?

Avengers 2: Age of Ultron - New Trailer

After Captain America: The Winter Soldier, the world needed an elite defense force to replace S.H.I.E.L.D. and deal with the remaining Hydra forces. Thor (Chris Hemsworth) returned to Earth after the events of Thor: The Dark World and Tony Stark (Robert Downey Jr.) helped bankroll and equip The Avengers after his own personal journey in Iron Man 3, bringing back together the six heroes from the first film and setting up shop in Stark Tower, now the headquarters of the Avengers.

When Avengers: Age of Ultron begins, the team has already been on numerous missions together against Hydra and all of the heroes have received upgrades to their field outfits and gear. The opening sequence features the Avengers attacking a Hydra base in Sokovia headed by Baron von Strucker.

Who is Baron Von Strucker?

Baron von Strucker (Thomas Kretschmann) in Captain America 2 Credits Scene

Baron Wolfgang von Strucker (Thomas Kretschmann) is a relatively new villain in the Marvel Cinematic Universe, first introduced at the end of Captain America: The Winter Soldier in a post-credits sequence. It is here where we're also introduced to Quicksilver (Aaron Taylor-Johnson) and Scarlet Witch (Elizabeth Olsen) who are in containment cells, seemingly out of control with super-powers we've never seen before. They are "enhanced" test subjects that Strucker has been experimenting on.

Strucker is a top Hydra official, seemingly the highest ranking one left according to the other Hydra officers in Agents of S.H.I.E.L.D.. His right-hand man is another scientist named Dr. List (Henry Goodman) who's frequently appeared in the television series, most notably the latest episode which serves as a prequel to Avengers: Age of Ultron.

The majority of season 2 of Agents of S.H.I.E.L.D. is built around killing off the remnants of Hydra and discovering the presence of enhanced people, something Hydra has made their top priority. We met the remaining "heads" of the Hydra organization earlier this season and by this week's episode titled "The Dirty Half Dozen" (the direct tie-in to Age of Ultron) only two remain: List and Strucker. Coulson's team attacks List's base in this episode to stop his experiments and find out where Strucker and Loki's Scepter are located (more on this down below) - and this is where we pick up in the prologue sequence of Avengers: Age of Ultron - in the Eastern European nation of Sokovia.

Understanding The "Enhanced"

Avengers: Age of Ultron Enhanced

Enhanced people are a growing regularity in the MCU even if they've not been a topic of the movies until now. Hydra is experimenting on them to understand how to harness, control and develop super-powered people, and S.H.I.E.L.D. (what's left of it on the TV show) wants to understand them in case they are dangerous. It's the current theme of Agents of S.H.I.E.L.D. and is quickly becoming a core element of the Marvel Cinematic Universe.

Quicksilver and Scarlet Witch ("the twins") show up right at the beginning of Avengers: Age of Ultron, and while their origins and source of power are not explained, they are referred to simply as "enhanced" people - the only two successful test subjects of Hydra to date.

In Marvel Comics, the pair are mutants (think X-Men) but for legal reasons, that explanation cannot be used in Avengers movies. Instead, Agents of S.H.I.E.L.D., has been laying the groundwork for enhanced people and introducing the idea of Inhumans - a hidden society of people with gifts living among regular folk here on Earth who are eventually getting their own movie in 2019.

Avengers: Age of Ultron does not touch on the topic of Inhumans but the official Avengers: Age of Ultron prelude comic "This Scepter'd Isle" does delve more into the backstory of the twins, revealing them to be part of the protests occurring in Sokovia. Strucker uses the protests as a way to recruit test subjects and the twins are the only ones who walk away from the experiments. Something about them is different...

The Return of Loki's Sceptre

Loki Scepter in Strucker's Posession

Several of the early films of the Marvel Cinematic Universe had a part in playing up the importance and power of the Tesseract ("Cosmic Cube" as it's also known as) and the alien object became the key MacGuffin of the first Avengers. Everyone wanted it but no one more than the Mad Titan Thanos - the big bad villain teased during the mid-credits button of The Avengers and villain behind-the-scenes in Guardians of the Galaxy.

Thanos gave Loki the powerful mind-controlling Scepter to use in his invasion of Earth to retrieve the Tesseract. After Loki was defeated by the Avengers however, Thor took him and the Tesseract back to Asgard, but the Scepter was strangely left on Earth in the hands of S.H.I.E.L.D.. So, what happened then when S.H.I.E.L.D. fell?

As explained in the official Avengers: Age of Ultron prelude comic "This Scepter'd Isle", Hydra agents inside of S.H.I.E.L.D. stole the Scepter and brought it to Baron von Strucker's Sokovia base where he used it to experiment on people in an attempt to create enhanced soldiers. Again, this is a storyline tying into season 2 of Agents of S.H.I.E.L.D. and the Scepter somehow unlocks abilities in Quicksilver and Scarlet Witch, but no one else Hydra experiemented on.

This week's episode of Agents of S.H.I.E.L.D. saw Coulson's team attack Hydra and discover Strucker's location in Sokovia and send it to Maria Hill (Cobie Smulders) for the Avengers to attack. The reveal here is that Coulson's main objectives all along have been to locate Strucker and the Scepter, and this episode solidifies the show's (and the prelude comic) role as a direct prequel to Avengers: Age of Ultron.

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Next: 10 Marvel Universe Easter Eggs Fans Wanted to See in Avengers 2

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Marvel’s Avengers: Age of Ultron stars Robert Downey Jr., who returns as Iron Man, along with Chris Evans as Captain America, Chris Hemsworth as Thor and Mark Ruffalo as The Hulk. Together with Scarlett Johansson as Black Widow and Jeremy Renner as Hawkeye, and with the additional support of Samuel L. Jackson as Nick Fury, Don Cheadle as James Rhodes/War Machine, Cobie Smulders as Agent Maria Hill and Stellan Skarsgård as Erik Selvig, the team must reassemble to defeat James Spader as Ultron, a terrifying technological villain hell-bent on human extinction. Along the way, they confront two mysterious and powerful newcomers, Wanda Maximoff, played by Elizabeth Olsen, and Pietro Maximoff, played by Aaron Taylor-Johnson, and meet an old friend in a new form when Paul Bettany becomes Vision.

The Avengers: Age of Ultron releases in theaters on May 1 2015, followed by Ant-Man on July 17 2015, Captain America: Civil War on May 6 2016, Doctor Strange on November 4 2016, Guardians of the Galaxy 2 on May 5 2017, Spider-Man on July 28, 2017, Thor: Ragnarok on November 3 2017, Avengers: Infinity War - Part 1 on May 4 2018, Black Panther on July 6 2018, Captain Marvel on November 2 2018, Avengers: Infinity War - Part 2 on May 3 2019 and Inhumans on July 12, 2019.