The Attack on the HYDRA Research Base was the final battle of the War on HYDRA. It pitted the Avengers against Wolfgang von Strucker's HYDRA cell just outside Novi Grad, Sokovia, with the intention to steal the Scepter from the base.
The battle ended with the deaths of many HYDRA operatives and personnel, including Dr. List, and Iron Man successfully acquiring the Scepter back to Avengers Tower, while Captain America incapacitated Strucker and placed him into NATO custody.
During an assault on a HYDRA research facility, Phil Coulson downloaded files that revealed the location of Wolfgang von Strucker and his illegal experiments. The files also revealed HYDRA was in possession of the Scepter wielded by Loki during the Battle of New York.
The Avengers immediately agreed to launch a surprise attack against HYDRA and permanently foil their plans.
The Battle Begins
Upon arriving in Sokovia, the Avengers wasted no time in carrying out their plan, unaware that Strucker was aware of their presence and had 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 soldiers were firing on Novi Grad, prompting the latter to send the Iron Legion to prevent any civilian casualties. Despite open hostility, the Legion 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.
In the forest, Hawkeye was preparing to fire an explosive arrow at a bunker when he was distracted by Pietro, who used his supersonic speed to taunt the former S.H.I.E.L.D. agent before taking off.
Hawkeye prepared to fire at Pietro only to be shot in the side by one of the bunker's cannons and seriously wounded. Black Widow came to his aid and tended to the wound as Hulk destroyed the bunker.
Pietro, meanwhile, attacked Captain America before making his way back to the base. Captain America relayed the information to Iron Man and followed Pietro as Thor helped Black Widow escort Hawkeye to the Quinjet.
Raiding the Facility
Locating a power source in the compound's northern section, Iron Man destroyed it and disabled the energy shield. He proceeded to subdue several soldiers with ease before locating List, who was in the process of deleting his files.
Back in the forest, the HYDRA soldiers surrendered and were taken into custody as Black Widow approached Hulk and hummed a lullaby that helped him calm down and transform back into Bruce Banner.
Retrieving the Scepter
While Iron Man discovered a secret passageway, Captain America confronted Strucker about his experiments only to be attacked by Wanda, who used her powers to throw him down a flight of stairs before fleeing the scene.
Quickly recovering from the attack, Captain America used his shield to subdue Strucker before informing the others of what transpired.
Meanwhile, Iron Man followed the secret passageway until he entered a laboratory housing, among other things, Chitauri armor and weaponry, partially-constructed robots and a Leviathan corpse. Locating the Scepter, he told Thor that he had "eyes on the prize" before moving 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, Iron Man turned around only to find he was alone. Suddenly, the Leviathan corpse came to life as Iron Man found himself standing in Chitauri Space, surrounded by his teammates' dead bodies and forced to watch the Chitauri invade Earth.
As Wanda and Pietro observed from the shadows, Iron Man snapped out of the trance and, after regaining his bearings, summoned one of his armor gauntlets before taking the Scepter.
Despite Pietro's protests, Wanda allowed Iron Man to leave in the hope that his actions would cause the Avengers' destruction.
- "It feels good, yeah? I mean, you've been after this thing since S.H.I.E.L.D. collapsed. Not that I haven't enjoyed our little raiding parties, but..."
"No, but this... This brings it to a close."
"As soon as we find out what else this has been used for. I don't just mean weapons. Since when is Strucker capable of human enhancement?"
"Banner and I'll give it the once over before it goes back to Asgard. Is that cool with you? I mean, just a few days 'til the farewell party."
- ―Iron Man, Thor, and Captain America
Deeply affected by his vision, Iron Man became determined to ensure global peace. After discovering an artificial intelligence hidden inside 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. Ultron proceeded to engage in a confrontation with the Avengers before making his way to Sokovia, creating a new body for himself and reconfiguring the robots to serve as his sentries.