“Who’s Ultron?” This was what Captain America himself, Chris Evans, said at Comic-Con last year after finishing his Hall H Winter Soldier presentation and heading backstage to do some press.

He had missed the finale of the Marvel Studios showcase, which revealed the Avengers sequel would be subtitled “Age of Ultron,” with a plot centered on that fearsome artificial-intelligence whose robotic face looks like a grinning metallic vampire skull.

Given the studio’s intense devotion to secrecy, both he and Scarlett Johansson had no idea who they’d be battling the next time the Avengers reunited.

It’s okay to forgive them a moment of uncertainty. There are plenty of fans out there who have come to this fantasy universe through the movies, and not the comic books.

So here, finally, is the movie’s answer to that question: Who IS Ultron?

HEAVY METAL MANIAC

The main thing to remember is, it’s very easy to go wrong trying to force others to do right.

That’s how Ultron, created as a force for protecting the world, becomes one of the greatest and most insidious threats to humanity.

He (the pronoun he prefers) was built to be both sentient and all-consuming in his quest for knowledge. He’s even programmed to feel emotions—although he gravitated toward rage rather than compassion. He could evolve and rebuild himself into bigger and stronger forms, but empathy was a struggle. Once he went rogue, Ultron was also hard to kill. Destroy one form, and he merely uploaded his consciousness into another form, like a robotic horcrux. (Hence all those other forms in the background of the First Look image above.)

In Age of Ultron, those origins details remain the same, but the person who created him has changed. In the comics, Ultron was built by scientist Hank Pym (a.k.a. the first Ant-Man). This time, he is the handiwork of Robert Downey Jr.’s Tony Stark—devised, essentially, as the ultimate drone, a way for Iron Man to abdicate his duties and offload them onto a fully mechanized protector.

The Avengers sequelbegins with Stark’s latest plan to fix the world: Ultron will be an all-seeing, all-knowing captain of a planetary police force known as the Iron Legion, a team of robotic beat cops who resemble blue-and-white versions of the Iron Man suit but have no human core—and less soul than a Carpenters album. If it all works out, the superheroes can rest easy.

IT DOES NOT WORK OUT

In a bid to give his creation a dose of humanity, Stark programs Ultron (performed and voiced by his Less Than Zero brat pack costar James Spader) with elements of his own personality—which proves to be Mistake No. 1.

The problem is that our new robot overlord is an absolutist, who inherits Stark’s cynicism, but not his sympathy. “It’s not the good version that could’ve come from [Stark’s] intellect and personality,” says Chris Hemsworth, returning as the hammer-hurling Thor. “It’s the bad son.”

In one of the opening scenes, the gang is celebrating with a swank party in the peak of Stark’s New York City skyscraper—formerly Stark Tower, it’s now the headquarters for the Avengers.

Stark sees the soiree as a chance for the Avengers to schmooze with the city’s powerbrokers in a post-S.H.I.E.L.D. environment, showing that they are not merely superpowered vigilantes. Half-filled glasses of wine, bottles of beer, and partially eaten plates of sushi and cookies litter the tables in this three-story marble and steel structure, constructed entirely within the soundstage—complete with a loading dock for the Quinjet and an upstairs laboratory for Stark and Banner (Mark Ruffalo) to blind themselves with science.

Thor (Chris Hemsworth) has put aside the cape and chest plate for a t-shirt and slacks, while Scarlett Johansson’s Black Widow has swapped her leathery catsuit for an empire-waist cocktail dress with a billowing skirt—which helps hide the actress’s real-life pregnancy.

Banner looks a little uptight in his purple button-down and tweedy jacket, while Jeremy Renner’s Hawkeye is in wallflower mode, lingering on the periphery by himself, ever the loner. Chris Evans’ Captain America is making small talk with former S.H.I.E.L.D. agent Maria Hill (Cobie Smulders). Most of the partygoers have left; now it’s just the old friends lingering.

AFTERPARTY AFTERMATH

That’s when all hell breaks loose. The Legionnaires have suddenly activated, and are inexplicably trying to kill the heroes. In the chaos, Ultron—in one of his first, metallic forms—announces his new plan to bring peace to the planet—by eradicating the most destructive thing that walks on it: humans.

In the midst of the assault scene, Downey finds himself suspended 50 feet above the set, riding piggyback on an actor in a motion-capture suit who will be digitally replaced with a hovering, hostile Legionnaire.

With no armor at his disposal, Stark grabs the only weapon handy—a fondue fork—and jams it into the robot soldier’s neck as they bang around the ceiling. “The deadliest fondue fork in all the land!” Whedon jokes. “From Odin’s melted cheese, I shall destroy thee!”

Even without their battle gear, the Avengers make short work of the traitorous, mechanical assailants. But Ultron is just getting started. He’s now thinking for himself … and they are not happy thoughts.

“I know you’re ‘good’ people,” he tells them. “I know you mean well… but you just didn’t think it through… There is only one path to peace… your extermination.” In the next shot, focused on the heroes’ reactions, the patronizing snarl of Ultron is delivered off camera by Whedon.

A few days before this scene was shot, however, Spader himself stood before the cast, hissing out the words like a bored prosecutor making the case against humanity.

“A lot of times when Ultron starts talking, it’s beautiful. It’s really intelligent stuff,” says Evans. “He’s out to do the things he wants to do because he’s disgusted with X, Y, and Z. You could probably sit down with Ultron and have a really intelligent conversation. He could blow your mind with his views.”

And then blow it for real?

Evans laughs. “Yeah, and then kill you.”