The film starts with The Avengers infiltrating a HYDRA base in Sokovia. Natasha Romanoff/Black Widow (Scarlett Johansson) drives in, kicking out goons while Clint Barton/Hawkeye (Jeremy Renner) fires his arrows. Thor (Chris Hemsworth) pounds his way with his hammer. Steve Rogers/Captain America (Chris Evans) rides in on his motorcycle, even flipping it against a van of HYDRA agents. The Hulk (Mark Ruffalo) smashes into the action. Finally, Tony Stark/Iron Man (Robert Downey, Jr.) flies to the HYDRA facility to break in and retrieve Loki's scepter. The base is guarded by an invisible shield. Tony summons his Iron Legion into town to protect civilians. Baron Wolfgang von Strucker (Thomas Kretschmann) rounds up his men with the intent to surrender, but his men urge him to send out their human enhancements, the twins - Pietro and Wanda Maximoff/Quicksilver and Scarlet Witch (Aaron Taylor-Johnson and Elizabeth Olsen). Pietro possesses superspeed while Wanda has telekinetic powers. He zooms through the forest and attacks The Avengers. Clint is wounded in the fight, while the others are all taken down. Nat calms The Hulk down so he may turn back into Bruce Banner.
Tony blows up the shield and gets into the facility. He exits his armor and walks through a secret passageway that leads to a room with recovered artifacts from the Battle of New York, including a gigantic Chitauri leviathan and some of Tony's scrapped armor. He spots the scepter as Wanda creeps up behind him. She peeks into Tony's mind and makes him witness one of his biggest fears - he is in the Chitauri realm and all of his friends are dead. He goes over to Steve, who says Tony could have saved them. Without a second thought, Tony grabs the scepter.
Upon returning to New York. Tony and Bruce discover an artificial intelligence in the scepter. With it, Tony hopes to initiate his Ultron program, which he intends to design for world peace since he knows that there may be some forces that even The Avengers could not control. Tony convinces Bruce to join him. They spend many hours awake, finding no program that works. They leave the lab for a party, and a failed experiment integrates itself to the program.
Ultron (James Spader) awakens, greeted by Jarvis (Paul Bettany). Jarvis explains Ultron's purpose to him. Ultron goes through a network of information regarding world events and any involvement of The Avengers in global matters. Seeing more harm than good, Ultron questions his purpose. He absorbs Jarvis's consciousness and begins to prepare himself a body.
The Avengers mingle at the party, with guests including Maria Hill (Cobie Smulders), James Rhodes/War Machine (Don Cheadle), Sam Wilson/Falcon (Anthony Mackie), and Helen Cho (Claudia Kim), a geneticist that aids Tony and Bruce in their research. Steve notices some some romantic tension between Bruce and Nat. Thor and Steve even get drunk with Stan Lee. After the guests have left, The Avengers play a game to see who can lift Thor's hammer. Tony can't do it even with his armored arm, or with Rhodey using his. Steve manages to budge it a little, to Thor's surprise. Suddenly, there's a loud screeching sound. Ultron shows up in a body made of scrapped Iron Man parts. He blames The Avengers for part of the chaos and destruction in the world, saying they are all puppets, while also bragging about killing Jarvis. He unleashes the warped Iron Legion bots at The Avengers. They all fight against the bots, destroying them before finishing off the Ultron body. He has already uploaded his consciousness elsewhere and has taken the scepter with him.
The twins encounter Ultron in an empty building, walking in a shiny new body. He explains his plan to take out The Avengers in order to make way for his own plans to bring peace to the world. The twins already have a grudge against Tony since it was his company's weapons that caused the destruction of their home and claimed the lives of their parents. Ultron knows he can use Wanda to manipulate the team as she has already seen into Tony's mind and knows that he created what he feared.
The rest of the team is upset with Tony and Bruce for creating Ultron, despite having no idea what he would do. They see a picture of Strucker, dead with "PEACE" written on the wall behind him in his blood. The Avengers learn that Ultron and the twins are going to Wakanda, Africa to find vibranium, the strongest metal.
The villains go to a shipyard in South Africa belonging to Ulysses Klaue (Andy Serkis). They get his supply of vibranium, and Ultron accidentally pulls off Klaue's arm. The Avengers arrive on the scene, minus Bruce waiting in the Quinjet. Iron Man fights Ultron, while Wanda gets inside the heads of Steve, Nat, and Thor. Steve sees himself in a 1940's dance hall, with Peggy Carter (Hayley Atwell) standing behind him, saying they can be together now. Nat sees herself in the facility where she was trained to be an assassin, and the harsh teachings of Madame B (Julie Delpy). Thor seems himself at a party on Asgard. He spots someone walking in a black cloak. Heimdall (Idris Elba) comes up to him, appearing blind and speaking crypitcally. Wanda almost gets Hawkeye, but he takes her out with an electric arrow. Pietro still zooms in and knocks Clint over. Wanda decides to make her move on Bruce, getting him angry enough to Hulk out and wreak havoc upon the city. Tony brings out his Hulkbuster armor (aka Veronica). The armor is powerful enough to beat down Hulk. Armed forces nearly intervene until Tony knocks Hulk out cold.
Bruce is back to normal, but he knows he's probably done serious damage. The world has already started turning on The Avengers, as Ultron had hoped for. Clint takes the team to a safe house...which is really his own home. He introduces them to his wife Laura (Linda Cardellini), along with his son and daughter. The couple are expecting their third child. The hallucinations brought on by Wanda continue to creep up in Thor's mind. He leaves to search for the answer to his vision. Steve and Nat are also reeling from their experience. Nat talks to Bruce about going off the grid and just hiding.
Ultron and the twins find Dr. Cho in Korea, carrying a cradle and a hefty supply of vibranium. Ultron uses the scepter to manipulate Cho into helping him create a new being to further his plan.
Laura calls Tony to help start the tractor, only to be surprised by Nick Fury (Samuel L. Jackson). He gives a few words of encouragement to the team to find a way to stop Ultron, despite their fears and concerns. Bruce realizes that Ultron is trying to evolve, and they need to find Dr. Cho.
Thor finds Dr. Selvig (Stellan Skarsgard) in London to aid him in his search for answers. They come across a cave. Thor goes into the water and experiences a vision of a large golden glove...the Infinity Gauntlet.
The new body is being prepped up for Ultron to upload his consciousness into. He breaks open the power source of the scepter. It's an Infinity Stone, the Mind Stone. He places it in the head of the body, allowing Wanda to read its mind. She sees a vision of global annihilation, which horrifies her. She and Pietro realize Ultron's true intentions, and Wanda breaks the scepter's hold off of Cho. Cho cancels the upload, just as The Avengers find them. Ultron hits Cho with a blast (non-fatally) and makes off with the cradle. Cap, Black Widow, and Hawkeye chase after the cradle. Cap fights Ultron briefly while Nat tries to make a grab for the cradle. The Avengers secure the cradle, but Nat gets taken by Ultron.
Tony tries to experiment on the synthetic body after he reveals that Jarvis is still alive and that he went underground to get away from Ultron. Cap and the twins come in to stop Tony from doing anything with the body, but Bruce helps him hold them off. Thor enters and pounds the cradle with his hammer, sending a powerful bolt of lightning that brings the body to life. It becomes The Vision (also Paul Bettany). Tony successfully uploaded a portion of Jarvis's program into the body. Thor explains the Infinity Stone's power to his friends, and how Vision can be essential in their fight. Vision is sort of uncertain about his purpose, but he knows that Ultron, despite feeling hurt over what he knows of the world, must be destroyed before he can cause pain to the rest of the world. For good measure, Vision picks up Thor's hammer, proving that he is indeed worthy.
The Avengers track Ultron to Sokovia, where he has amassed an army of robots that share his consciousness. He has placed a vibranium core beneath the city, setting it off and causing part of the earth to float to the sky. If it drops from a high enough altitude, it can destroy the world like a meteor. The Avengers fight back, guiding civilians to safety as Fury, Maria, and other SHIELD agents show up in the Helicarriers, taking the civilians to safety, while Rhodey shows up in his War Machine armor to fight the bots. Hawkeye convinces a frightened Wanda to fight back and become an Avenger. She does so and uses her powers to help destroy the Ultron-bots.
Vision finds Ultron, in his new Prime form, and prevents him from connecting to other bodies. Angered, Ultron fights against Vision and Thor. Wanda and Iron Man attempt to hold off the core while also fighting off the bots. Bruce finds Nat and breaks her out of her cell. She kisses him and pushes him down a hole so he can Hulk out. They join the fight and get civilians to safety.
Tony realizes that they need to destroy the core, even if it means obliterating the city and possibly killing him and his teammates. He joins with Vision and Thor as they deliver a powerful blast to Ultron. Hulk shows up and knocks him far away, causing the other bots to retreat. Ultron hijacks a jet and shoots at Hawkeye as he runs to save a boy. Pietro, having been moved by the heroism of SHIELD and The Avengers, intervenes and takes the shots to save Hawkeye. He dies, and Wanda senses his death, causing her to fall in despair. Hulk knocks Ultron out of the jet and into a train. Wanda finds the Ultron Prime body and rips his core out, so that he knows how it feels to die as a human would.
Tony fires a powerful blast to the core, accompanied by Thor's lightning. Together, they destroy the core and the entire chunk of the city. Hulk sits in the jet as Nat reaches out to him. Knowing he can't be with her without possibly hurting her, he turns off Nat's camera and flies away to parts unknown. Meanwhile, Vision encounters the last Ultron body. Vision acknowledges Ultron's fear of death, and he obliterates the villain.
In the aftermath, The Avengers establish a new base in upstate New York. Clint's baby is born, and he is partially named after Pietro. Thor returns to Asgard to take care of business while Tony and Steve part ways. Nat misses Bruce, while Fury assures her that he will be out there somewhere for her to find. Steve comes in and takes her so that they can train some friends. They gather War Machine, Falcon, Scarlet Witch, and Vision to be the newest Avengers.
Midway through the credits, we get a look at the Infinity Gauntlet, without any of the Stones. Thanos (Josh Brolin) puts it on and vows to find the Stones himself.
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