Time Heist

"No, you accidentally survived. It's a billion to one cosmic fluke and now you wanna pull off... what do you call it?" "A time heist."

- Tony Stark and Scott Lang

Operation Time Heist was a planned mission organized by the Avengers and aimed at undoing the Decimation by going back in time and retrieving the Infinity Stones from major historical events, resulting in the final battle against Thanos who traveled from 2014 to 2023.

Ambush on Thanos
"We get the stones. We use them to bring everyone back."

- Captain Marvel

In 2018, the Avengers, Captain Marvel, and Nebula travel to the Garden to retrieve the Infinity Stones from Thanos. When they arrive, they are met with a heavily crippled Thanos who destroyed the Infinity Stones to keep his work intact. After realizing there is no way to bring back the vanished, Thor decapitates Thanos and they return to Earth, completely broken.

Scott Lang's Return
"That must have been a very terrible five years for you." "It wasn't five years, it was more like five hours."

- Natasha Romanoff and Scott Lang

In 2023, the Avengers became further torn apart after the fact that they could not reverse the effects of the Decimation. In a San Francisco warehouse, a rat accidentally walked over the controls of the Quantum Tunnel, which was housed in Luis' Van, and released Ant-Man from the Quantum Realm. Ant-Man then reunited with his daughter and went to the New Avengers Facility to find the Avengers. Once there, Lang discussed the Quantum Realm with Steve Rogers and Natasha Romanoff, explaining the physics behind the Realm and how five years had passed for the Avengers while it was only five hours for him. With that in mind, Lang suggested using the Quantum Realm to go back in time to prevent the Decimation from occurring.

Planning the Heist
"Five years ago we lost. All of us. We lost friends. We lost family. We lost a part of ourselves. Today, we have a chance to take it all back. You know your teams, you know your missions. Get the stones, get them back, one round trip each. No mistakes. No do-overs. Most of us are going somewhere we know. That doesn't mean we should know what to expect. Be careful. Look out for each other. This is the fight of our lives... and we're gonna win. Whatever it takes."

- Captain America

With Hank Pym gone and a short supply of Pym Particles left, the Avengers went to Tony Stark for assistance but Stark turned them down, having moved on and not wanting to risk his daughter's life. Hulk attempted to run Quantum tests on Scott Lang which proved to be ineffective. Not long after, Stark returned having figured out how to safely and accurately travel the Quantum Realm to reach different time periods.

With Stark's help the Avengers created a new Quantum Tunnel and Advanced Tech Suits as they planned out their heist, deciding to split into three group to search of the different stones. Lang however warned that due to the short supply of Pym Particles they can only make one travel back and forth each so there's no room for any mistakes.

Theft of the Mind Stone
Aware that the Mind Stone was in the Scepter that Loki used during the events of the Battle of New York, Iron Man, Captain America, and Ant-Man set out to retrieve it and the Tesseract which contained the Space Stone as well. The Scepter was retrieved by Captain America from Jasper Sitwell and Brock Rumlow after muttering Hail Hydra to Jasper, which was more than enough to convince them to give him the case with the Scepter, leaving them stunned.

Theft of the Time Stone
Aware that the Time Stone was in possession of the Masters of the Mystic Arts, the Hulk arrived at the New York Sanctum where the Ancient One was defending the Sanctum from the invading Chitauri. Banner, however, was unaware that Doctor Strange was not part of the Masters of the Mystic Arts in 2012 and was still a doctor. The Ancient One caught on to the current state of affairs and initially refused to give the stone to Banner, fearing that her new reality may be defenseless without the Time Stone, and that the Masters of the Mystic Arts swore to protect the Stone with their lives and giving it to him would break the oath. Banner then told her that Strange already broke the oath during the Battle of Titan when he gave the Time Stone to Thanos. Knowing that Strange would only do this if he genuinely had no choice, she eventually gave it to Banner who promised that all the stones would be returned to their proper places in time afterwards and that they would survive.

Theft of the Space Stone
Ant-Man was tasked with incapacitating the 2012 Tony Stark so that the future Stark could get the case with the Tesseract in the confusion. Ant-Man shrunk inside the 2012 Stark's Arc Reactor in order to sabotage the device, which caused him to endure a heart attack. The suitcase was then kicked to the future Stark disguised as a security guard. As he was about to take the stairs up, the 2012 Hulk came crashing through, angered about using the stairs to go down. The case found its way to the 2012 Loki who used the Tesseract to get away. The failure caused Stark to make the decision to go back to the 1970's where the Tesseract, as well as extra Pym Particles, could be found in Camp Lehigh.

Theft of the Reality Stone
Thor and Rocket Raccoon arrived in 2013 Asgard on the day the Dark Elves attacked. Thor was distracted by the sight of his mother Frigga, forcing Rocket to get the Reality Stone from Jane Foster himself, only to be chased by the Einherjar. Rocket eventually met up with Thor and Frigga. Before they went back to the present, Thor was able to summon Mjølnir to him, happy that he was still worthy.

Theft of the Power Stone
After being dropped off by Black Widow and Hawkeye with the ship's pod on Morag,  War Machine and Nebula waited for Star-Lord, who would steal the Orb containing the Power Stone. The two then trailed Quill, who agreed that he was an idiot. War Machine then knocks out Star-Lord and Nebula steals the tool that Star-Lord would use to gain access to the room containing the orb. Nebula then used her cybernetic arm to get the orb. The two then proceeded to transport themselves back to the present. However, in the process, Nebula was unable to transport after something went off in her head.

Kidnapping of Nebula
The 2014 Nebula was on a mission with the 2014 Gamora when Thanos summoned them to debrief them on their mission to retrieve the Power Stone with Ronan the Accuser. However, Nebula began feeling pain and began playing a memory from the future Nebula which prompted Thanos to bring them to a room with Ebony Maw, where he revealed that the implants in both Nebulas were interacting with each other which allowed both to access the others' memory files. This allowed Thanos to see the future where he ends up being decapitated by Thor. Thanos stops Ebony Maw from killing Nebula as he discovered a way to accelerate his plans. Nebula on Morag rushed to the pod to contact Clint and Natasha to warn them that the 2014 Thanos was now aware of their plans. but was captured. The 2014 Nebula then went to the future in order to help bring Thanos and his army to 2023.

Sacrifice of Natasha Romanoff
Hawkeye and Black Widow arrived on Vormir where they meet the Red Skull and are told about the stone requiring a sacrifice. The two spent time on the edge debating on who should be the one to die. Hawkeye volunteered, asking Romanoff to tell his family that he loves them. Romanoff attacked Barton, telling him to say it himself before rushing off to the edge. Hawkeye then chased her down and the two have a scuffle. They then fall off the cliff. with Barton using a grapple to hang on to while holding on to Natasha. She then allows herself to fall to her death. The sky lights up and Barton wakes up in a pool of water with the Soul Stone in his hand in the same way that Thanos got the stone in 2018. He then makes his way back to the present.

Aftermath
Everyone, including the 2014 Nebula, arrived back at the facility where they learn of Romanoff's sacrifice. The original Avengers commemorate her life at the lake, with Banner throwing a bench into the sky. They accept that reversing the Decimation will not bring everyone that didn't die at the hands of Thanos back, proceeding to create a new gauntlet. Thor volunteers to do the snap but is turned down due to his condition. Banner decides to do it, citing that the Stones emit gamma radiation, meaning that he would be the only one to survive using it. Everyone braces themselves in the event that something goes awry, with Stark putting the building on lockdown. Meanwhile, the 2014 Nebula arrives and uses the quantum machine to bring 2014 Thanos and his ship to the present. The Avengers were too occupied to notice the commotion, even when a part of the roof had been destroyed. After some difficulty, Banner snaps his fingers and every victim of the Decimation is restored. This is proven when Barton gets a call from his wife and Scott Lang notices birds on a tree that were not there before.