In 2015, the Avengers—Iron Man (Robert Downey Jr.), Captain America (Chris Evans), Thor (Chris Hemsworth), Hawkeye (Jeremy Renner), Black Widow (Scarlett Johansson) and the Incredible Hulk (Mark Ruffalo) reassemble to track down and eliminate the remnants of HYDRA. Their latest mission brings them to the location of the alien scepter that Loki utilized in 2012 when he brought the Chitauri to Earth. The scepter is now in the possession of Baron Wolfgang von Strucker (Thomas Kretschmann), a HYDRA operative who has been using energy drawn from the alien weapon to power advanced weaponry, as well as attempting to empower living humans.
The movie opens with the Avengers launching an assault on Strucker's base in a fortress outside the city of Novi Grad, Sokovia. As the team are fighting their way past the guards around the base's perimeter, two of Strucker's successful experiments join the battle: Wanda Maximoff (Elizabeth Olsen), a young woman who possesses powers of telekinesis and mind manipulation, and her twin brother Pietro (Aaron Taylor-Johnson), who possesses superspeed. Pietro rushes outside and joins the battle with the incoming Avengers. Clint is wounded when he's grazed in the side by a blast from a gun nest due to Pietro distracting him, forcing Natasha to abandon the fight to tend to him.
While Thor gets Clint off the battlefield and Natasha uses a lullaby to coax the Hulk out of Bruce, Tony and Steve manage to penetrate the fortress. Steve captures Strucker as he is preparing to flee, but not before having a brief run-in with Wanda. Meanwhile, Tony kills Dr. List, Strucker's top underling, as he's trying to delete files on their work. While Tony has JARVIS download the data for preservation, he finds a secret laboratory underneath the fortress in which Strucker's people have been taking a Leviathan apart, and the scepter, sitting in plain sight. As Tony walks up to the scepter, he doesn't notice that Wanda has crept into the lab. As he's about to grab the scepter, she walks up behind him and casts a spell on him, implanting a vision into his mind. In his vision, Tony sees his teammates dead or dying while the Chitauri launch an even larger invasion of Earth. After a few moments, he snaps out of the vision, and realizes it was all a dream. Pietro makes it downstairs, and is confused by the fact his sister is just letting Tony walk away. Wanda grins mischievously as Tony summons part of his armor and grabs the scepter.
With Strucker now in NATO custody, HYDRA has suffered a great blow and the organization is left in a state of discord. Returning to New York with the scepter, Tony and Bruce begin studying the weapon, having convinced Thor to wait a few days before he takes it back to Asgard. Their research leads to the discovery of a net of neurons that Tony realizes could be reconfigured into an artificial intelligence stored inside the scepter's gem. The two scientists extract it for use in Ultron, a peacekeeping program that Tony has designed to allow his Iron Legion to operate independently and safeguard Earth under the direction of a more advanced A.I. than JARVIS.
After three days, the project is seemingly met with little success. However, as the Avengers throw a party in the penthouse at Avengers Tower to celebrate their recent victories, Ultron (James Spader) suddenly activates. Somehow already self-aware despite Tony's work not having made it that far ahead, Ultron browses the Internet and immediately determines that the only way to save Earth is to eradicate humanity, which he deems as the prime factor in the world's turmoils. JARVIS tries to reason with Ultron, but the more powerful A.I. appears to destroy him and, feeling he needs a body, he downloads himself into the Iron Legion armors. Towards the end of the Avengers' party, the team, joined by Maria Hill, James Rhodes, Sam Wilson and geneticist Helen Cho (who tended to Clint's injuries) hold a small contest to see who can lift Mjølnir.
Their contest is interrupted when Ultron suddenly takes control of the Iron Legion and attacks them. During the fight, Ultron almost kills Cho but mysteriously spares her life. While the Iron Legion drones are all eventually dispatched, one of them flees across the Atlantic with the scepter to Sokovia and takes up shop in Strucker's old HYDRA base. There, Ultron uses the HYDRA technology stored there to begin construction of an army of robotic drones as well as his own mobile unit.
In the aftermath of the fight, the other Avengers confront Tony about his secrecy. Meanwhile, with a new body built, Ultron seeks out the Maximoff twins. He intends to use them, and Wanda's powers specifically, to drive a wedge between the Avengers. This is something the twins are onboard with, as they have a vendetta against Tony after missiles built by his company killed their parents in 1999; to that end, Wanda had willingly allowed Tony to make off with the scepter with hopes that his fear of letting the world die would lead to him constructing his own undoing, which in this case appears to be Ultron.
Ultron, with the twins and an army of drones at his side, hits up several former HYDRA bases for resources, killing anyone who resists while using Wanda to neutralize everyone else. Word of what he's doing gets back to the Avengers when he kills Strucker in his cell, writing the word "PEACE" on the wall in Strucker's blood. Through some detective work, the team determines that Ultron is interested in buying vibranium from an arms dealer named Ulysses Klaue (Andy Serkis), who stole a large quantity of it from the African country of Wakanda many years ago.
Ultron and the Maximoffs infiltrate Klaue's ship, with the twins going ahead to strongarm Klaue. Klaue is not fazed by the twins' efforts to intimidate him, and demands to speak to their new employer, prompting Ultron to reveal himself. Ultron wires a 10 figure sum of money to Klaue's various offshore bank accounts in exchange for Klaue's stockpile. But the transaction goes awry at that point when Klaue recognizes a phrase Ultron uses as something Tony used to say to him a long time ago, and tries to back out. In a fit of rage, Ultron slices off Klaue's left arm.
At that point, the Avengers ambush them. Ultron engages Tony while leaving the others to fend off the twins, Ultron's sentry drones, and Klaue's henchmen. The fight is very short-lived, as Wanda hexes Thor, Steve, and Natasha, giving them vivid hallucinations that render them catatonic. Thor finds himself in a hellish party in Asgard, where a blinded Heimdall gleefully claims that they are all dead in a very disturbing way. Natasha has flashbacks to her training in the Red Room, where she had to execute a bound and blindfolded person as well as undergo surgery that left her sterile. Steve has a dream about seeing Peggy Carter again at a party where she asks him to dance and celebrate that the war is over. The only one to avoid being brainwashed is Clint, who shoots Wanda with an arrow that sticks to her forehead and shocks her, forcing her to retreat with Pietro.
Pietro gets Wanda off the ship. Wanda is undeterred by the setback, and noticing Bruce alone in the Avengers' quinjet, she brainwashes him. Whatever vision she gave Bruce, he turns into the Hulk, makes his way to Johannesburg, and begins causing mayhem. After blowing up Ultron's current body, Tony calls in "Veronica," his Mark XLIV armor designed to contain the Hulk. After a lengthy fight, Tony is able to contain the Hulk, but not before several buildings are destroyed. The resulting public backlash forces the team to go into hiding.
The team takes refuge with Clint's family in central Iowa, where they meet his pregnant wife Laura, son Cooper and daughter Lila. Thor temporarily leaves, traveling to the mystical "Water of Sights" with Dr. Erik Selvig to determine the meaning of his hallucination. Natasha and Bruce make plans to run away and start a new life together when Ultron is defeated. They are encouraged to mend fences by the arrival of Nick Fury, who convinces them to form a plan to stop Ultron. Noting Ultron's fondness for the human body despite its limitations, Bruce realizes Ultron plans to use the vibranium to build himself a new body made out of synthetic tissue, like that which Cho used to heal Clint's wounds.
Tony becomes intrigued when Fury notes that Ultron hasn't managed to access any nation's nuclear launch codes. It's clear Ultron has an unseen enemy who is keeping him at bay. To find out who this is, Tony travels to a Nexus internet hub in Oslo, Norway for leads, and comes to the realization that Ultron's engaging with JARVIS, who is still alive and well.
Ultron makes his way to Cho's laboratory in Seoul, South Korea, and demands that she use the vibranium in conjunction with synthetic tissue from her regeneration cradle to create a more powerful body for himself, with the scepter's gem as its primary power source. To ensure her cooperation, Ultron uses the scepter to brainwash Cho. As Ultron transfers his consciousness into the synthetic body and Cho implants the gem into its forehead, Wanda becomes curious, realizing that she'll now be able to read Ultron's thoughts. She touches the cradle, and immediately jerks away in horror when she sees an image of an asteroid hitting the Earth, indicative of his real plans to destroy humanity. Realizing that Ultron used them, Wanda quickly snaps Cho out from the scepter's influence to disrupt the transfer process and flees with Pietro. Realizing what's happened, Ultron wounds Cho and kills her assistants, before he and his sentries leave with the regeneration cradle in tow, planning to take it back to Sokovia.
As Ultron and his drones drive to the airport in a semi truck, they are ambushed by a team comprised of Steve, Clint, and Natasha. Steve engages Ultron in a fight atop the truck, while Natasha manages to get into the trailer, and Clint provides assistance from the quinjet. The fight eventually takes them into a moving train, while Ultron's sentries attach themselves to the truck's trailer and lift it skyward. Onboard the train, Ultron gains the upper hand on Steve, and begins beating him up. He is stopped by the surprise arrival of Pietro and Wanda, who attempt to wall him in. Undeterred, Ultron fires a unibeam blast in Pietro's direction, killing the train's driver, then takes off in pursuit of the flying trailer. Natasha plants explosives in the trailer to destroy it and transfers the cradle to the quinjet, but is snatched by Ultron in the process. Meanwhile, the runaway train crashes through a bumper block and careens out of control through the streets. After a moment of prodding from Steve, the twins leap into action. Pietro jumps off and runs ahead to clear pedestrians out of the way while Wanda applies a concentrated burst of telekinetic energy to brake the train to a stop. As they compose themselves, Wanda is dismayed to learn from Steve that the cradle is on its way to Tony, and suspects that he has an ulterior agenda on hand.
Ultron takes Natasha back to Strucker's old fortress and throws her in a cell, seeking to show off while he uses the vibranium to construct his next and most durable body. Unbeknownst to him, she manages to access some communication equipment, allowing her to send a message to Clint on her location. Meanwhile, in New York, Tony and Banner discover that JARVIS in fact wasn't killed when Ultron attacked him; he actually went underground and has been keeping Ultron from accessing any nation's military codes. Since JARVIS has always been a benevolent AI, Tony thinks that their best chance is to use him to power the new body.
They have almost completed the process when Steve arrives with Pietro and Wanda, demanding they shut it down. A fight breaks out, which is resolved when Thor arrives, summons lightning with Mjolnir, and directs it into the cradle. Moments later, the body inside the cradle bursts out, revealing itself to be a 6'4" naked metal man (Paul Bettany). Wanda is quickly smitten with the new being, while Thor explains that the gem in the scepter is the Mind Stone, one of the six Infinity Stones, which Thor saw in his hallucination, and its power could be advantageous against Ultron. The being, who takes to calling himself Vision, quickly establishes that he is trustworthy by effortlessly lifting Mjolnir.
The Avengers and their new allies return to Sokovia, where Ultron has used the remaining vibranium samples and Chitauri anti-gravity technologies that Strucker was studying to build an anti-gravity drill that lifts a large part of Novi Grad into the sky. His intent is to crash it into the ground to replicate a meteor strike, causing a mass extinction event. While Bruce frees Natasha, the Avengers evacuate civilians from the city, and engage Ultron's army of robots while Vision tracks down Ultron and burns him out of the Internet, meaning he can only transfer himself to his drones. Ultron doesn't take his loss lying down, and out of spite, he activates the drill, causing the city to ascend into the sky, with no way to evacuate those still in it. Fortunately, not all is lost, for Fury and Rhodey turn up with a helicarrier, and dispatch flying lifeboats to ferry evacuees off the rock.
Bruce and Natasha enter the fray and the team converges on the drill, located in the church where Ultron first met the Maximoff twins. A battle ensues as the team engage Ultron's drones, wiping out most of them. Ultron's primary body also engages them, but Vision is able to separate him from the main battle, using a laser emanating from the Mind Stone to drive him back. With assist from Tony's unibeams and Thor's lightning, they inflict severe damage to the primary body, melting the right side of his face, and the Hulk then tosses him away as he tries to monologue.
With most of the drones taken care of, the Avengers work on getting the last evacuees onto the lifeboats while Wanda volunteers to guard the drill. Unbeknownst to them, Ultron lands near the Avengers' quinjet and decides to hijack it as a last-ditch resort. He takes to the sky and goes on a strafing run, firing the quinjet's gatling guns at full auto, disrupting Natasha's efforts to give her lullaby to Hulk and taking aim at Clint, who's forced to go back into town to find a young boy who is still trapped.note Clint shields the boy from the approaching gunfire when at the last second, Pietro runs in front of him, intercepting the bullets at the cost of his own life. Devastated by the loss of her one remaining family member, Wanda collapses to her knees in grief, letting out a burst of energy that vaporizes the majority of the remaining drones on the ground.
As Ultron prepares to fly off, Hulk deposits Natasha on the helicarrier, then leaps onto the quinjet and rips Ultron from the cockpit, sending him plummeting into the city center, where he crash-lands in an abandoned streetcar. As he lays there, immobilized, Wanda enters the streetcar, stewing with rage. He begs her to leave for her own safety, to which she responds by tearing his power core out of his chest, shutting him down.
With Wanda away from her post, one of the remaining drones crawls over to the drill and turns the key to deactivate it. Novi Grad begins plummeting towards the Earth, and Steve and Clint are forced to flee aboard the Helicarrier alongside the evacuated citizens. Realizing Wanda isn't on-board, Vision flies into the falling city and rescues her, then flies out while Tony fires his Unibeam and Thor summons lightning to reflect off the drill's vibranium core. The ensuing explosion shatters the rock into smaller pieces, which rain down on a nearby lake. Hulk, seeking peace after the battle, flies off in the quinjet to parts unknown.
But Ultron is not quite dead yet, for his consciousness still survives in a severely damaged drone that Steve had knocked off the floating rock at the start of the battle. After getting Wanda to the Helicarrier, Vision intercepts the Ultron sentry just as it has finished crawling out of the crater that used to be Novi Grad. The two have a brief exchange of ideals, before the drone tries to attack Vision, forcing Vision to obliterate him with his laser.
With the day saved, the Avengers establish a new training facility in upstate New York, overseen by Fury, Cho, Maria Hill, and Selvig. Natasha assumes Bruce is dead because the quinjet cannot be tracked in stealth mode, and silently mourns for him, but Fury assures her that he is likely still alive. Believing the Mind Stone is safe with Vision, Thor returns to Asgard to learn more about the Infinity Stones, while Clint and Tony also leave the team to go back to their personal lives. The movie ends as Steve and Natasha assemble and begin training their new team of Avengers, comprised of Wanda, Vision, War Machine, and Falcon.
In a mid-credits scene, Thanos, dissatisfied by the failures of his pawns, dons the Infinity Gauntlet and vows to retrieve the Infinity Stones personally...
