Marvel's The Avengers is the third-highest grossing movie of all time, and it's the rare superhero movie that managed to please hardcore comic-book fans and mainstream viewers alike. So yes, it's safe to say that a whole lot of people are interested in learning everything they can about its upcoming sequel, Avengers: Age of Ultron.

Since the film's title was announced at Comic-Con last year, we've learned a few things about the Avengers sequel. Joss Whedon, who masterminded The Avengers, will return as writer and director for Age of Ultron, and the rosters of superheroes from the original is also slated to return. But beyond those bare details, much of the film is still shrouded in mystery. What do we know about Avengers: Age of Ultron? A guide:

1. Avengers: Age of Ultron will hit theaters in May 2015
May 1, 2015, to be exact. That's a few weeks after Fast & Furious 7, and a few months before the release of the summer's other two big superhero movies: Fantastic Four, which will reboot the popular comic-book franchise, and Marvel's Ant-Man. Warner Bros.' Batman vs. Superman, which was originally scheduled for release in July 2015, has since been bumped to May 2016 — so Marvel is roundly expected to dominate 2015's summer blockbuster lineup.

2. Almost everybody who starred in The Avengers will be back
It wouldn't be an Avengers sequel without all the Avengers, and fortunately, Marvel has managed to lock the whole gang down: Iron Man (Robert Downey Jr.), Captain America (Chris Evans), Thor (Chris Hemsworth), The Hulk (Mark Ruffalo), Black Widow (Scarlett Johansson), and Hawkeye (Jeremy Renner) will all reprise their roles in the sequel.

But that's not all: Most of the supporting cast from The Avengers will be back too. Familiar faces include Samuel L. Jackson and Cobie Smulders as two high-level S.H.I.E.L.D. agents, and Don Cheadle as Tony Stark's best friend James Rhodes, a.k.a. War Machine.

The one major face that probably won't return? Agent Coulson (Clark Gregg), a fan-favorite character who was killed off in The Avengers but revived for the TV spinoff Agents of S.H.I.E.L.D. The plot of The Avengers hinged on Coulson's death, and the Avengers themselves have no idea that their former ally has been revived — but Whedon has suggested that Coulson will sit this mission out. "Right now it's not something that I'm pursuing because I have so much going on in Avengers 2," said Whedon at San Diego Comic-Con last year. "Finding out that Coulson is alive would be an entire B story, and I already have too much movie."

3. James Spader will star as Ultron

(Jason Merritt/Getty Images, Marvel)

As its title indicates, the Avengers sequel will hinge on a new villain: Ultron, an artificially intelligent robot who will be played by James Spader. In the comics, Ultron was created by Ant-Man, but eventually turned on his creator and set his sights on ruling Earth. Ultron is one of the most powerful villains in the Marvel universe: Super strong, super smart, super fast, and virtually indestructible. If that wasn't frightening enough, he's also capable of upgrading himself, which means that he continues to grow more threatening and unstoppable all the time. Whedon has said that the film version of Ultron will be somewhat different — Ant-Man won't be his creator, and his powers will be diminished — but by and large, he sounds a lot like the Ultron fans know from the comics.

It's a logical choice for a villain that will raise the stakes after the massive battle at the climax of The Avengers — but why James Spader, who doesn't exactly scream "indestructible supervillain?"

"Spade was my first and only choice," explained Joss Whedon. "He's got that hypnotic voice that can be eerily calm and compelling, but he's also very human and humorous. Ultron is not HAL. Spader can play all of the levels. He's the guy to break the Avengers into pieces." Marvel president Kevin Feige later explained that Spader would have "more than a voice role," with motion capture being used for his face and body movements — and in September 2013, Spader confirmed that Marvel has taken "very extensive photographs, head scans, body scans, and all kinds of things" as he prepares to take on the role.

4. Thomas Kretschmann will play secondary villain Baron Wolfgang von Strucker

(David Buchan/Getty Images For TheWrap, Marvel)

Ultron may be getting all the press, but earlier this month, news quietly broke that Avengers: Age of Ultron would have a secondary villain: Baron Wolfgang von Strucker, who will be played by Thomas Kretschmann (most recently seen as Abraham Van Helsing on NBC's Dracula). No announcement has been made about the role Von Strucker will play in the film's overarching story, but in the comics, the villain is the leader of HYDRA — a Nazi group introduced in Captain America: The First Avenger (and briefly referenced as an ongoing threat in TV's Agents of S.H.I.E.L.D.).

5. New heroes Scarlet Witch and Quicksilver will be introduced


In addition to the two new villains, The Avengers sequel will be introducing a pair of brother and sister superheroes: Scarlet Witch (played by Elizabeth Olsen) and Quicksilver (played by Aaron Taylor-Johnson).

The duo's introduction isn't really a surprise — both characters have been Avengers in the comics for decades, and Joss Whedon had previously revealed that he would introduce a "brother-sister act" in the sequel — but they should provide an intriguing shakeup to the chemistry of the Avengers lineup. Scarlet Witch is a telekinetic who can manipulate psychic energy; Quicksilver can run at super speed.

In the comics, Scarlet Witch and Quicksilver are the children of X-Men villain Magneto — but a dispute with 20th Century Fox, which holds the film rights to the X-Men franchise and all associated properties, will reportedly keep Marvel from referencing that part of their backstory in The Avengers sequel. (Confusingly, Quicksilver will also appear in 20th Century Fox's upcoming X-Men: Days of Future Past — but played by Evan Peters, and wholly disconnected from Taylor-Johnson's version of the character in The Avengers sequel.)

It's not yet clear how Scarlet Witch and Quicksilver will fit into the movie's storyline, but both Olsen and Taylor-Johnson have expressed enthusiasm for their roles. "[Scarlet Witch] has more things wrong with her than any character I've ever played," said Olsen, which is a rather telling statement from the star of Silent House and Martha Marcy May Marlene. "[Quicksilver] has real anger frustration – I like that," said Taylor-Johnson. "I always though it'd be quite funny if you saw him eating loads and people asked him why, and he'd explain it's because he's burning so much energy all the time."

6. Age of Ultron will work as a standalone movie
If all this backstory has your head spinning, relax: According to Joss Whedon, Avengers: Age of Ultron won't require knowledge from the comics — or even the movies that preceded it. "The events of [the upcoming Captain America: The Winter Soldier] will definitely affect the world of Avengers 2," said Whedon. "But at the end of the day, I have to make my movie assuming that people will only have seen the first one, or possibly not even seen the first one. I can't assume that everybody went to see [Thor: The Dark World, Captain America: The Winter Soldier], and Iron Man 3 in between. I have to go from one movie to the next and be true to what's happened, but not be slavish to it."

7. Age of Ultron will be the end of Marvel's "Phase Two"
Marvel's "Phase One," which began with 2008's Iron Man, ended with The Avengers in 2012. Consequently, last year's Iron Man 3 marked the beginning of what Marvel has called "Phase Two," which is followed by last year's Thor: The Dark World and the other two upcoming Marvel films, Captain America: The Winter Soldier and Guardians of the Galaxy (which is far more loosely tied to The Avengers' narrative).

But Marvel has even larger plans on the horizon. In a 2011 interview, Kevin Feige speculated that Phase Two "will culminate, god willing, in The Avengers 2," which will be followed by the release of the Paul Rudd-starring Ant-Man — presumably the kick-off to a Phase Three — just two months later.