In the Eastern European country of Sokovia, the Avengers – Tony Stark, Steve
Rogers, Thor, Bruce Banner (Hulk), Natasha Romanoff, and Clint Barton – raid a Hydra
outpost led by Baron Wolfgang von Strucker, who has been experimenting on
humans using the scepter previously wielded by Loki. They encounter two of
Strucker's experiments – twins Pietro, who has superhuman speed, and Wanda
Maximoff, who can manipulate minds and project energy – and apprehend Strucker,
while Stark retrieves Loki's scepter.
Stark and Banner discover an artificial intelligence within the scepter's
gem, and secretly use it to complete Stark's "Ultron" global defense
program. The unexpectedly sentient Ultron, believing he must eradicate humanity
to save Earth, eliminates Stark's A.I., JARVIS, and attacks the Avengers at their
headquarters. Escaping with the scepter, Ultron uses the resources in
Strucker's Sokovia base to upgrade his rudimentary body and build an army of
robot drones. Having killed Strucker, he recruits the Maximoffs, who hold Stark
responsible for their parents' deaths by his weapons. Together, they go to the
base of arms dealer Ulysses Klaue in an African shipyard to obtain Wakandan vibranium.
The Avengers pursue them, but Wanda subdues the Avengers with haunting visions,
causing the Hulk to rampage until Stark stops him with his anti-Hulk armor.
A worldwide backlash over the resulting destruction, and the fears Wanda's
hallucinations incited, send the team into hiding at Barton's safehouse. Thor
departs to consult with Dr. Erik Selvig on the meaning of the apocalyptic
future he saw in his hallucination, while Romanoff and Banner plan to flee
together after realizing a mutual attraction. However, Nick Fury arrives and
encourages the team to form a plan to stop Ultron. In Seoul, Ultron forces the
team's friend Dr. Helen Cho to use her synthetic-tissue technology, together
with vibranium and the scepter's gem, to perfect a new body for him. As Ultron
uploads himself into the body, Wanda is able to read his mind; discovering his
plan for human extinction, the Maximoffs turn on Ultron. Rogers, Romanoff, and
Barton find Ultron and retrieve the synthetic body, but Ultron captures
Romanoff.
The Avengers fight amongst themselves when Stark secretly uploads
J.A.R.V.I.S. – who is still operational after hiding from Ultron inside the
Internet – into the synthetic body. Thor returns to help activate the body,
explaining that the gem on its brow – one of the six Infinity Stones, the most
powerful objects in existence – was part of his vision. This "Vision"
and the Maximoffs accompany the Avengers to Sokovia, where Ultron has used the
remaining vibranium to build a machine to lift a large part of the capital city
skyward, intending to crash it into the ground to cause global extinction.
Banner rescues Romanoff, who awakens the Hulk for the battle. The Avengers
fight Ultron's army while Fury arrives in a Helicarrier with Maria Hill, James
Rhodes and S.H.I.E.L.D. agents to evacuate civilians. Pietro dies when he
shields Barton from gunfire, and a vengeful Wanda abandons her post to destroy
Ultron's primary body, which allows one of his drones to activate the machine.
The city plummets, but Stark and Thor overload the machine and shatter the
landmass. In the aftermath, the Hulk, unwilling to endanger Romanoff by being
with her, departs in a Quinjet, while the Vision confronts Ultron's last
remaining body.
Later, with the Avengers having established a new base run by Fury, Hill,
Cho, and Selvig, Thor returns to Asgard to learn more about the forces he
suspects have manipulated recent events. As Stark and Barton also leave, Rogers
and Romanoff prepare to train new Avengers: Rhodes, the Vision, Sam Wilson, and
Wanda.
In a mid-credits scene, Thanos, dissatisfied by the failures of his pawns,
dons a gauntlet and vows to personally retrieve the
Infinity Stones.
