During an assault on a HYDRA facility, S.H.I.E.L.D. Director Phil Coulson downloaded files that revealed the location of Wolfgang von Strucker and his illegal experiments. The files also revealed that HYDRA was in possession of the Scepter wielded by Loki during the Battle of New York.
Sometime later, the Avengers agreed to launch a surprise attack against HYDRA and foil their plans for world domination.
The Battle Begins
Upon arriving in Sokovia, the Avengers carried out their plan, unaware that Strucker had learned of their presence and alerted his men.
Iron Man attempted to enter the compound only to be repelled by an energy shield. J.A.R.V.I.S. analyzed the shield and mentioned that Strucker's technology was more advanced than the other bases they had raided.
Inside the compound, Strucker deployed the tanks as List proposed that they send Pietro and Wanda Maximoff as a distraction. Strucker refused, confident that his men were capable of emerging victorious.
Meanwhile, J.A.R.V.I.S. alerted Iron Man that the troops were firing on Novi Grad. The latter then sent the Iron Legion to evacuate the area. Despite some resistance, the Legion successfully managed to get several civilians to safety.
Inside the base, Strucker rallied his men with a speech before privately telling List that he planned to surrender and carefully instructing the scientist to erase all data from the base's computers. Before List could take any action, however, the two realized the twins were missing.
Meanwhile, in the woods, Hawkeye was preparing to destroy a bunker with an explosive arrow when he was distracted by Pietro, who used his supersonic speed to distract the former S.H.I.E.L.D. agent before taking off.
Hawkeye prepared to aim his bow at Pietro, but was shot in the side by one of the bunker's turrets and gravely wounded. Romanoff came to his aid while the Hulk destroyed the bunker.
Pietro then attacked Captain America before making his way back to the base. Captain America relayed the information to Stark and followed Pietro as Thor helped Black Widow get Hawkeye to the Quinjet.
Raiding the Facility
Having located a power source in the compound's northern section, Stark destroyed it and deactivated the energy shield. After neutralizing several HYDRA soldiers, Stark found Doctor List, who was erasing his files.
After fatally shooting List with a repulsor ray, Stark placed his armor in automated sentry mode before tasking J.A.R.V.I.S. with copying the remaining files and sending them to Avengers Tower.
In the forest, the HYDRA soldiers surrendered and were taken into custody as Romanoff approached Hulk and recited a lullaby that helped him calm down and transform back into Bruce Banner.
Retrieving the Scepter
While Stark discovered a secret passage, Rogers confronted Strucker about his experiments only to be attacked by Wanda, who quickly fled the scene. Rogers subdued Strucker and informed the others.
He found the Scepter and moved forward to take it from its resting place. Before he could do so, however, Wanda crept up behind him and used her powers to implant a vision into his mind.
Sensing Wanda's presence, Stark turned around to find that he was alone. Suddenly, the Leviathan corpse came to life as Stark found himself in Chitauri Space, surrounded by his teammates' lifeless bodies and forced to watch the Chitauri invade Earth.
Snapping out of his trance, Stark summoned one of his armor gauntlets and took the Scepter as Wanda and Pietro watched from the shadows.
Deeply affected by his vision, Stark became even more determined to ensure global peace. After discovering an artificial intelligence hidden within the scepter's gem, he used it to create the Ultron Program.
However, the program became self-aware and convinced itself humanity was responsible for preventing peace. After a confrontation with the Avengers, Ultron made his way to Sokovia and created a new body for himself before reconfiguring the sentinels created by HYDRA.