After Lila lands a bullseye, Clint goes to retrieve the arrow; but when he turns around, she is nowhere to be found. He doesn't recognize the telltale ashy residue... and because he's looking for her, he doesn't see Laura, Cooper and little Nate get Reduced to Dust either. As the sounds of birds and wildlife are diminished by half, he dashes about the field in increasing panic, the latest victim of the mad tyrant Thanos, who succeeded in gathering the Infinity Stones and using their immense power to wipe out half of all life on Earth, and throughout the entire universe.
The Marvel Studios logo rolls, but half the heroes in it are missing.
Tony Stark, Iron Man, finds himself lost in space with Nebula, a surly cyborg and one of the only two remaining Guardians of the Galaxy, as his only company. Their future is looking increasingly dim: the Benatar is out of fuel and will soon lose life support. Tony uses his helmet to record what he believes to be a final message to his fiancée, Pepper Potts. As he is prepared to drift off into the hereafter, however, he sees a golden glow approaching the ship, at the center of which is an angel with flaxen hair: Carol Danvers, Captain Marvel.
With Carol's help, Tony and Nebula return to Earth and are reunited with the remaining Avengers. Pepper is also waiting for him, having been spared from Thanos's Badass Fingersnap. In the three weeks since, Earth's governments (and presumably those of every other planet) have been reeling. Unfortunately, after all he had been through, Tony is in no shape, physically or mentally, to be of any help: after having lost so completely to Thanos, and watching those he cared for die around him, he has no fight left in him.
The other Avengers, however, are not ready to give up hope. Captain Steve Rogers, Natasha "Black Widow" Romanoff, James "War Machine" Rhodes, Thor, Rocket Raccoon, Bruce Banner, and Carol: they all want nothing more than to undo the damage Thanos caused. Thankfully, they have a clue to his whereabouts: a distant planet that Nebula states Thanos wanted to retire to after seeing his plans through. Armed with a newly-repaired Benatar and the Title Card, they head out into space.
They find Thanos in a shack, his arm grievously injured from The Snap. The Avengers restrain him and demand the location of the Infinity Stones, but Thanos revealed that he has, using their own power, destroyed them, so that neither he nor anyone else can use them. Thor, fueled by grief and frustration, decapitates the Mad Titan — a meaningless victory that solves nothing.
Humanity has adapted to life after The Snap. Natasha collaborates with Rhodes, Rocket, Nebula, Carol, and the Wakandan general Okoye to help maintain a semblance of order on Earth and beyond, while Steve heads a support group for those who had lost loved ones to Thanos. Even though Steve espouses the virtue of moving on, the fact remains that neither he nor Nat can bring themselves to do so.
Meanwhile, in San Francisco, a rat scurrying around an impound lot for food happens across a van that once belonged to a security consultant company called X-CON, containing a Quantum Tunnel developed by the scientist Hank Pym. The rat accidentally activates the tunnel, and in so doing, allows Ant Man, Scott Lang, to escape the Quantum Realm, where he was trapped. A Fish out of Temporal Water, he scrambles around the desolate city, trying to find out what happened.
Scott makes his way to the Avengers compound with a radical idea. Though five years have passed since the end of Ant-Man and the Wasp, in which The Stinger showed his entire Cast Herd getting Snapped, for him it's only been five hours. Time in the Quantum Realm flows differently than in the physical realm. Thus, it stands to reason that the Quantum Realm could be used to power a Time Machine, which would allow the Avengers to Set Right What Once Went Wrong.
Such a plan would require a genius mind to see through. Steve knows of one: Tony, who had married Pepper and had a daughter, Morgan, since last they met. Steve, Natasha, and Scott visit Tony to propose their idea, but Tony refuses, having answered the Call to Agriculture (or at least domesticity) with alacrity.
There is another genius who could help that Steve and Scott reach out to: Dr. Bruce Banner. Since they last met, Banner has integrated his personality, and now exists as "Professor Hulk," combining Hulk's strength and Bruce's sound mind, a gentle green giant. Bruce agrees to help try to build the time machine.
In the end, however, Tony relents: finding an old picture of himself and the dear friend he lost to Thanos, a kid from Queens named Peter Parker, gives him the impetus he needs to look into whether a time machine could be built. To his own astonishment, he manages to make a breakthrough in the design of the time machine. Discussing the matter with Pepper, he attempts to display a Refusal of the Call, but his wife asks if he could ever rest when there was more he could do. So Tony goes to the Avengers compound, and at long last buries the hatchet with Steve Rogers. He offers his aid in what Scott has dubbed the "Time Heist" under one condition: whatever they do, whatever changes they make to space and time, it cannot endanger his family.
As the greatest (remaining) scientific minds of Earth work on the time machine, the Avengers reach out to others for aid. Natasha travels to Tokyo and finds Clint, now operating as "Ronin," a vigilante hunting down the world's surviving criminal population. The death of his family had driven him onto a Roaring Rampage of Revenge, but the prospect of seeing them again, no matter how remote, sees him return to the fold. At the same time, Hulk and Rocket go to a Norwegian town, "New Asgard", and reunite with Thor, who had become a drunken recluse whiling time away with friends he made on Sakaar. The mention of Thanos's name turns the drunk-yet-jolly god of thunder into a Shell-Shocked Veteran, as he feels personally responsible for failing to land the killing blow on Thanos during the Battle of Wakanda. But Thor Odinson is a simple man at heart; all it takes is the promise of more beer.
Barton offers to do the trial voyage through the time machine. He lands at his house and succeeds at bringing a baseball mitt home with him — answering a critical question about whether any plans to steal the stones from the past can actually succeed. As he does, the others discuss the rules of time travel, particularly the possibility of Continuity Snarls, Grandfather Paradoxes, etc, and the fact that the Avengers could change their own past — or could they?
- Professor Hulk: I don't know why everyone believes that, but that isn't true. Think about it: If you travel to the past, that past becomes your future, and your former present becomes the past! Which can't now be changed by your new future!
Scott: ...So Back to the Future's a bunch of bullshit?!
With the time machine complete and ready, it is time to formulate a plan of action: the Avengers will travel back in time to three different points in history to collect the Infinity Stones. Steve, Tony, Scott, and Bruce will go to New York in the year 2012, when the Space, Mind, and Time Stones can be found in the Tesseract, Loki's scepter, and the Sanctum Santorum, respectively. Thor and Rocket head to Asgard in the year 2013, when the Reality Stone materialized as an entity called "the Aether" in the body of Thor's then-lover Jane Foster. Finally, Nebula, Rhodes, Natasha, and Clint will go into the year 2014 to retrieve the Power and Soul Stones from the planets of Morag and Vormir.
Because the entire process is powered by Pym Particles, whom Hank Pym is not un-Snapped enough to synthesize more of, the Avengers can only make one jump back in time and another jump home. There is only one shot. They must make it count.
- Steve's party arrived in the year 2012, when Manhattan was under siege by Loki and his army of Chitauri.
- Bruce immediately made his way to the Sanctum Sanctorum and met the Ancient One, the Sorceress Supreme before Dr. Strange. Bruce asked her for the Time Stone, but the Ancient One rebuked him, citing the Time Stone as instrumental to Earth's protection against invasions from other dimensions. The two reinforced the idea of Story Branching as the metaphysics underlying time travel: while obtaining the Time Stone would save Bruce's continuity, it would doom the Ancient One's, and she was — quite reasonably — unwilling to condemn her timeline to The End of the World as We Know It. Bruce, in return, volunteered that his continuity's Avengers would return their borrowed Stones, when they were done, to the exact moment they borrowed them, thus saving these Alternate Timelines as well as their own. He also mentioned that Strange had already given the Stone to Thanos during the events of Infinity War. The Ancient One was shocked: she knew Strange was destined for greatness... so if he'd given the Time Stone to Thanos, it must've been for a very good reason. Realizing that they were apparently on the one timeline in 14 million that will result in Thanos's ultimate defeat, the Ancient One entrusted the Time Stone to Bruce.
- Steve, Tony, and Scott infiltrated the Avengers Tower to obtain Loki's scepter and the Tesseract. Steve retrieved the scepter from HYDRA agents disguised as S.H.I.E.L.D. personnel; the strike team had their suspicions, but he managed to put them to rest by whispering, "Hail HYDRA." As Steve left, however, he encountered his younger self, who mistook him for Loki in disguise. Steve managed to distract 2012 Captain America by mentioning that Bucky was still alive, and subdued him with Loki's scepter.
- Meanwhile, Scott and Tony followed Loki and the Avengers to get the Tesseract. Scott, shrinking himself, disabled 2012 Tony's arc reactor as a distraction, and made off with the Tesseract; however, they were derailed when 2012 Hulk showed up in a temper and threw the scene into disarray. Loki, seeing a chance, grabbed the Tesseract and immediately teleported away into a TV series, Loki (2021).
- Loki's escape with the Tesseract threw everything into jeopardy, but Steve and Tony had one last idea: travel further back in time and retrieve the Tesseract from S.H.I.E.L.D.'s base of operations in 1970. This would've used up the last of their Pym particles, but Tony knew his father was there studying the Tesseract, and knew from his father that Dr. Pym was there as well. Thus, while Scott Lang takes the Mind Stone back to 2023, Steve distracted a younger Dr. Pym to steal some of his particles, and from a distance, spotted S.H.I.E.L.D. Director Peggy Carter, his lost lenore from some time ago. Meanwhile, Tony encountered the last person he expected to see: Howard Stark, his father. Tony had a heartfelt conversation with his unaware father about family, achieving the closure he never got before time travel, before he and Steve returned with their quarries.
- Thor and Rocket arrived in Asgard in 2013. Rocket retrieved the Reality Stone from Jane Foster. Thor had intended to help him, but was overcome when he realized it was the day his mother Frigga had / was going to die. This created even more tension when they accidentally ran into each other. Frigga realized that this Thor was not from her time, but was nonetheless proud of the man her son had become, and assured him that even though he failed to stop Thanos before, the measure of a man, a hero, was their ability to keep trying to make things right. Thor, his Heroic Resolve renewed, took one additional souvenir with him: his warhammer, Mjölnir, which had yet to be destroyed by Hela in this when.
- Meanwhile, Nebula's group arrived in the year 2014. The team split up: Natasha and Clint dropped Rhodes and Nebula on Morag, and then took the Milano to Vormir.
- On Morag, Rhodes and Nebula found Quill amidst the opening credits to Guardians of the Galaxy and had no trouble giving him a Tap on the Head while he was distracted. Retrieving the Power Stone was similarly easy: Nebula invalidated the trap by not caring if her cybernetic arm took damage. Rhodes took the Stone back with him to the present, but Nebula began to seize up: her neural implants had inadvertently networked with her 2014 self, who, along with 2014 Gamora, was meeting 2014 Thanos to discuss his plan. 2014 Nebula began to replay memories from 2023 Nebula, and Thanos saw that he is destined to succeed. He tasked Nebula with capturing her future self and going forward in time in her stead: the Avengers will have all of the Stones gathered in one place, making his plans to achieve "balance" in the universe that much easier. 2023 Nebula, however, told 2014 Gamora of her future fate, and the two joined forces to stop Thanos.
- On Vormir, Clint and Natasha found the keeper of the Soul Stone, the man once known as the "Red Skull", displaced across the cosmos by the Space Stone and now serving as the Soul Stone's eternal guardian. At the top of the mountain, the Red Skull recapped the whole "sacrifice who you love most" thing: for Heterosexual Life-Partners Clint and Natasha, this meant each other. Clint, who had already lost everything, was ready to throw himself off to let Natasha take the Stone; Natasha, the Tykebomb Anti-Hero, preferred to atone via her death. The result was a preposterously heartfelt Duel to the Death, as the two friends scrambled to see who could commit Heroic Suicide first as an Act of True Love. Natasha lost and/or won, leaving a brokenhearted Clint with the Soul Stone.
Clint's phone buzzes as Laura, Back from the Dead, calls him. Scott looks out to see the restored world, hearing — for the first time since the Cold Open — the tweeting of birds.
- Scott: Guys... I think it worked!
2014 Nebula has been busy. Disguising herself as 2023 Nebula, she returned to 2023 with the Avengers. While the others were working on the Stark Infinity Gauntlet, she commandeered their Quantum Tunnel and used it to pull Thanos's flagship, Sanctuary II, forward into 2023. It starts by saturation-bombing the place into oblivion.
In the rubble below, Rocket, Rhodes, Bruce, and Scott are nearly buried alive. As they struggle to get back to the surface, 2014 Nebula leads her father's forces to find Clint, who has the Infinity Gauntlet in hand. 2023 Nebula, Gamoranote by her side, guns down the woman she used to be. Meanwhile, Steve, Tony, and Thor engage Thanos, who has realized his plan was flawed: even if he succeeded, there would be those who would attempt to undo his balance. His new solution is to go full Omnicidal Maniac: to break the entire universe down and remake it from scratch, perfectly balanced... with no one any wiser to the sacrifices needed to make it.
Steve, Tony, and Thor give it everything they have. This time, Thanos doesn't have any Infinity Stones to help him... but the Mad Titan is still a match for Earth's Mightiest Heroes. Even as Steve proves worthy enough to wield Mjölnir, Thanos matches him blow for blow. He incapacitates Thor and Tony, and his assaults shatter Steve's shield. Thanos lands his entire army, and Steve, alone, drags himself to his feet, straps the remnants of his shield to his arm, and prepares for his final stand.
He's interrupted by a radio message: "On your left."
A portal opens, through which steps King T'Challa of Wakanda, the Black Panther ...and he's not alone. More portals open, bringing with them allies from all across the universe!
Sam Wilson, the Falcon!
Bucky Barnes, the Winter Soldier!
Wanda Maximoff, the Scarlet Witch!
Hope van Dyne, the Wasp!
Peter "Star-Lord" Quill, Drax, Groot, and Mantis: the Guardians of the Galaxy!
Pepper Potts in her own Iron Man suit, "Rescue"!
Valkyrie leading the combined forces of Asgard and the Ravagers!
King T'Challa's armies, with Princess Shuri, General Okoye, and the united tribes of Wakanda!
Doctor Stephen Strange, the Master of the New York Sanctum Sanctorum, and Wong with every Master of the Mystic Arts!
Peter Parker, the amazing Spider-Man!
From the rubble emerges the remaining Avengers: Ant-Man, War Machine, Rocket and the Hulk!
The stage is set for the battle that will determine the fate of the universe, and Steve gives the marching orders to his friends:
- Steve Rogers: "AVENGERS... assemble."
- Tony: Hey. You said one out of fourteen million, we win, yeah? Tell me this is it.
Dr. Strange: If I tell you what happens... it won't happen.
As Strange battles to keep the water at bay, he and Tony make eye contact. Hands shaking, Strange holds up one finger.
Thanos has the gauntlet, and even Iron Man's desperate last-minute attempt to dissuade him is not enough. Triumphant — "I am... inevitable." — he snaps his fingers.
All he gets is an anticlimatic metallic clank.
Tony reveals that the Stones are in his possession, stolen from the Stark-Tech Infinity Gauntlet. He attaches them to his own glove before responding with the words that launched a franchise:
"And I... Am... Iron Man."
Thanos watches in silence as his forces are Snapped out of existence. He sits, dignified in death, as ash takes him too.
The war is over. The heroes have won... but at great cost: the power of the Infinity Stones have ravaged Tony's body. So wounded he can barely speak, Anthony Edward Stark dies, surrounded by the family he made, the Super Team that grew up around him, the Cinematic Universe he founded, and the world that he saved.
Thor appoints Valkyrie the new queen of New Asgard before leaving with the Guardians of the Galaxy, who have made it their mission to figure out what has become of the 2014 Gamora. She did not vanish with the rest of her father's army (a scene showing her survival is in the DVD Extras) but hasn't been seen since the battle; her friends are hopeful that she is somewhere in the universe. Bruce confides in the others that despite a conscious effort to bring Natasha home as well when he used the Gauntlet, it seems her sacrifice is irreversible. Wanda, who has noticed that Vision likewise did not return, comforts him, assured that wherever their fallen friends are, they know that what they did was meaningful.
Meanwhile, Steve takes it upon himself to go back in time to return the Infinity Stones, along with Mjölnir, back to their proper places in time. He vanishes into the past...
...but doesn't return!
...at least, not as the Avengers expected him to.
They find Steve sitting on a nearby bench, significantly aged. Steve explains that after finishing his task, he decided to take The Slow Path. As Sam ponders how the world will go on without Captain America, Steve assures him that it won't, giving a gift to Sam: his shield. The honor, and the responsibility that comes with it, are immense... but Sam accepts Steve's call, becoming the new Captain America.
Steve won't go into specifics about what exactly he did after finishing his final mission, but he admits that he did finally fulfill a promise he made many years ago: that he would share a dance with the love of his life, Peggy Carter.