The Time Heist was a planned mission organized by the Avengers, aimed at undoing the Snap 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.
In 2018, the Avengers, Captain Marvel, and Nebula traveled to the Garden to retrieve the Infinity Stones from Thanos. When they arrived, they found a heavily crippled Thanos, who had destroyed the Infinity Stones to keep his work intact. After realizing there was no way to bring back the vanished, Thor decapitated Thanos and the Avengers returned to Earth, completely broken.
Scott Lang's Return
- "Time works differently in the Quantum Realm. The only problem is, right now, we don't have a way to navigate it. But what if we did? I can't stop thinking about it. What if, we could somehow control the chaos, and we could navigate it? What if there was a way to enter the Quantum Realm at a certain point in time but then exit at another point in time? Like... Like before Thanos."
- ―Scott Lang[src]
By 2023, the Avengers became further torn apart after the fact that they could not reverse the effects of the Snap. Tony Stark had gone to live his life with his wife and daughter, Steve Rogers had begun to help others cope with therapy, while Black Widow continued to run the Avengers with Okoye, Rocket Raccoon, War Machine, and Captain Marvel. In a San Francisco warehouse, a rat accidentally walked over the controls of the Quantum Tunnel, which was housed in Luis' Van, and released Scott Lang from the Quantum Realm. Lang 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 Snap from occurring.
Planning the Heist
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. With Stark refusing to help, Banner was contacted by Steve Rogers, Natasha Romanoff, and Scott Lang to see if he could help with the test on time traveling, before Banner revealed his new condition to the three.
After some persuasion, Hulk attempted to run some tests on Lang by letting him go through the Quantum Realm which proved to be ineffective due to sending time through Lang instead of sending Lang through time. Not long after, Stark returned with the newly invented Time-Space GPS which allows for safe and accurate traveling through the Quantum Realm to reach different time periods. After a test run on Clint Barton, who was brought in by Romanoff after the time heist idea was hatched, proved successful, the Avengers planned the many possibilities in a session, concluding which points in time would be most effective to reclaim the Infinity Stones.
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 groups to search for 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.
Quest for the Infinity Stones
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 Sitwell, which was more than enough to convince them to give him the case with the Scepter, leaving them stunned.
Aware that the Time Stone was in possession of the Masters of the Mystic Arts, 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 afterward and that they would survive the ordeal.
Ant-Man was tasked with incapacitating the 2012 version of Tony Stark so that the future Stark could get the case with the Tesseract in the confusion. Ant-Man shrunk inside the 2012 version of Stark's Arc Reactor in order to sabotage the device, which caused him to endure a mild heart attack. The suitcase was then kicked to the future Stark disguised as a security guard. As he was about to take the case and meet up with Rogers, the 2012 version of Hulk came crashing through the stairwell, angered about using the stairs to go down instead of the elevator. The case found its way to the 2012 version of Loki, who used the Tesseract to get away. When Stark and Lang reunited with Rogers and explained their failure, Stark proposed the idea to go back to the 1970s where the Tesseract, as well as extra Pym Particles, could be found in Camp Lehigh. Stark and Rogers decided to go together and tasked Lang with taking the Mind Stone back to present day.
Thor and Rocket Raccoon arrived in 2013 Asgard on the day the Dark Elves attacked. Thor quickly lost his composure at the sight of his old home, and while Rocket did his best to keep him focused, Thor was distracted by the sight of his mother Frigga, who died the same day, forcing Rocket to get the Reality Stone from Jane Foster himself. After sneaking into her room and extracting it, he was chased by the Einherjar. Rocket eventually met up with Thor and Frigga, who had been talking outside. Before they went back to the present, Thor was able to summon Mjølnir to him, happy that he was still worthy.
After being dropped off by Black Widow and Hawkeye with the ship's pod on Morag, War Machine and Nebula waited for the 2014 version of 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 knocked out Star-Lord and Nebula stole 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.
The 2014 version of Nebula was on a mission with the 2014 version of 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 stopped Ebony Maw from killing Nebula as he discovered a way to accelerate his plans. Nebula on Morag also saw a memory file of her past self, altering her that Thanos was now aware of the Avengers' plan and rushed to the Benatar’s pod to contact Clint and Natasha to warn them, but was captured. The 2014 Nebula then stole the Advanced Tech Suit and disguised herself as the present Nebula and went to the future in order to help bring Thanos and his army to 2023.
Hawkeye and Black Widow arrived on Vormir where they meet the Red Skull and were 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. Instead, 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. When Barton emerged victorious, he flung himself off the cliff, but Romanoff jumped on him, and then they both fell off the cliff with Barton using a grapple to hang on to while holding on to Natasha. She then allowed herself to fall to her death. The sky lit up and Barton woke 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 made his way back to the present.
Everyone, sans 2023 Nebula, who was subdued and impersonated by her 2014 self, and Natasha Romanoff, arrived back at the facility, where they all celebrated their victory. However, this was cut short, when Clint Barton fell to his knees, causing everyone to see that Romanoff was not there, realizing that she had died during the mission. The original Avengers gathered outside to mourn their friend, where Barton made it clear that Romanoff could not be brought back. They all then made a promise to not let their friend's death go meaningless, so they started working on controlling the Infinity Stones to reverse the Snap.
They proceeded to create a new gauntlet that could harness the power of the stones. Thor volunteered to wield the gauntlet, but was turned down due to his unstable state. Banner informed them that he had to do it, stating that the stones emitted Gamma Radiation, so he would be the one with the best chance to survive it. Everyone then suited up and braced themselves in the event that something went awry, with Stark putting the building on lockdown. Meanwhile, the 2014 version of Nebula, who took the present-day version's place, used the Quantum Tunnel to bring the 2014 version of Thanos and his ship to the present, crashing through the roof and taking its place in the sky above the facility.
The Avengers were too occupied to notice the commotion, even though a part of the roof had been destroyed. After some difficulty containing the gauntlet's power, Banner said that he was okay and was able to snap his fingers, restoring every victim of the snap. They realized it worked when Barton got a call from his wife and Scott Lang noticed birds on a tree that were not there before. However, this was cut short, when the facility was destroyed by Thanos, causing the Avengers to be separated: Barton landing with the gauntlet as Stark, Steve Rogers and Thor confronted Thanos.