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The story opens with an explanation of where the movies and the ‘actual’ MCU diverge: Dr. Strange gave Thanos a fake Time Stone, and as a result, the heroes are able to destroy the Infinity Stones. Not long after, however, they learn that the stones have reformed in our world, and Thanos is preparing to make another attempt at claiming them. But an unexpected obstacle arises—namely, the fact that most of the heroes have counterparts in our world, and they cannot be in the same universe at the same time. Fortunately, the ghost of Loki agrees to help, in exchange for a return to corporeality. Traveling to our world (as a spirit, he is not restricted by the counterpart rule), he possesses Tom Hiddleston, his dimensional equivalent, and explains the situation to him. Hiddleston agrees to cooperate under a few conditions, all of which Loki accepts. Loki and Dr. Strange then swap the spirits of the original six Avengers, plus Ant-Man and the Wasp, with those of their counterparts—their actors.
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Upon arriving in our world, the team is given a quick briefing by Loki, who also provides them with props from the films, which have been enchanted to function like the real deal. As the stones reformed in different places, the group splits up to find them:
-Tony and Bruce locate the Power Stone in Northern Australia, where Bruce encounters a young man who helps him reconcile with the Hulk.
-Natasha and Steve track down the Time Stone (in Brazil), while musing on the events of Endgame and how they differ from what happened in their universe.
-Clint and Thor travel to Florida, where they find the Mind Stone in the hands of a research team studying children with learning and developmental disorders (the latter are high-functioning). Having accidentally enhanced the children under their care, the researchers gladly give up the stone and agree to keep the whole thing a secret, and Clint gives them some advice on how to help the children with their powers and their disabilities.
-Scott and Hope discover the Space Stone has made its way into the possession of an influential British politician, who’s been secretly engaging in a less than legal relationship. In the process of retrieving the Stone, they expose the affair, resulting in some nasty fallout.
-Loki is able to retrieve the Reality Stone from eastern Africa with little trouble, though he does end up having a long conversation with Hiddleston and what path he’ll take once he’s alive again.
Once the first five stones are retrieved, they track down the Soul Stone, finding it in Afghanistan. While Natasha mentions how Thanos acquired the stone, and what happened in Endgame, they find that it has already accepted a sacrifice, though they are unable to find out who died. As it turns out, however, they got the last stone not a moment too soon—Thanos and his army have arrived.
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Meanwhile, in the heroes’ universe, Doctor Strange gives the actors a quick lowdown. He also reveals his intention to revive not just Loki, but Quicksilver, Vision, and several other heroes. The actors volunteer to travel to the land of the dead in order to retrieve the fallen ones. Heimdall’s spirit greets them at the entrance, warning them that since Hela was slain, the land of the dead is without a ruler, and consequently has fallen into chaos. During their journey, they encounter many dead villains like Obadiah Stane and Darren Cross, who try to claim their souls. Luckily, they also receive aid from the ghosts of friends like T’Chaka and the Ancient One. They also find Balder, who has been Hela’s prisoner since her banishment. After instating Balder as the new ruler of the land of the dead, the actors return to the land of the living with the spirits of Quicksilver, Vision, the Warriors Three, and Gamora in tow, just in time to see a horde of Mindless Ones arriving.
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Doctor Strange is surprised by the Mindless Ones’ presence due to his bargain with Dormammu, but after a quick check, he realizes that there is someone else behind their presence, though this being is not currently present nor affiliated with Thanos. Receiving a heads-up from Loki, Strange swaps himself and a number of the other heroes with their counterparts to aid the battle against Thanos. Wong provides said counterparts with a quick breakdown before setting to work on resurrecting the fallen.
In the actors’ universe, Loki gets Hiddleston to safety before releasing him and returning to the heroes’ universe. The heroes equip themselves with transformed props (provided by Loki at Strange’s request), before joining in the final battle, which plays out similarly to the final battle of Endgame. But instead of one person using the Infinity Stones to turn Thanos and his army to dust, the original six Avengers work together, splitting the stones’ power amongst them and thus surviving with no permanent damage (although they do need to rest).
Once this is over, the heroes take a while to recover before they and their actors (who defeated the Mindless Ones) switch back. The story ends with the heroes in their everyday lives—which includes Bruce as Professor Hulk, Thor and Loki helping Gamora get back to the Guardians, Wanda happily reuniting with Pietro and Vision, and Tony with Pepper and Morgan—before they receive a message: “Avengers, Assemble.”
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In a ‘post-credits scene,’ it’s revealed that the actors who were swapped still retain a measure of their counterparts’ abilities and skills. As Tom Hiddleston, Benedict Cumberbatch, Chris Evans, and Robert Downey Jr. discuss this, they also mention evidence that there may be more to their world than they previously knew. RDJ asks what they’re going to do, and Evans gives him a knowing look.