Yes, the plot of Avengers: Doomsday really has leaked online in full. With reshoots planned, we can expect some changes and, as always, there's likely a fair bit that's been lost in translation as it's been passed from one person to the next.
However, we've done some digging and have verified with at least two trustworthy sources that what you're about to read below is the real deal.
The plot of Avengers: Doomsday is detailed from start to finish. While there are one or two points that are a little fuzzy, all signs point to the Russo Brothers delivering a suitably epic movie that, alongside Avengers: Secret Wars, will wrap up the Multiverse Saga in style. Needless to say, major spoilers follow from this point on.
To learn more about what's to come in Avengers: Doomsday, click the "Next"/"View List" buttons below...
The Movie's Opening Scene
Avengers: Doomsday begins in the reality that Tobey Maguire's Spider-Man calls home. He's in the midst of a battle with Deadpool and Wolverine, who have been sent there by Beast with a device capable of destroying this world (similar to how the TVA would "prune' timelines).
Wolverine manages to set the device and stays behind, perishing alongside the web-slinger. Deadpool, meanwhile, returns to his world, while we see Professor X watching on with sadness, as another planet falls.
However, the X-Men had no choice, as, similar to what we saw in Jonathan Hickman's New Avengers run—which led directly into Secret Wars—you either save one reality or allow both to be destroyed in the Incursion.
Steve Rogers Returns
The movie takes us back to Tony Stark's funeral in Avengers: Endgame and follows Steve Rogers into the past, where he chooses to build a life with Peggy Carter. They have a son, Jim, and receive a visit from Loki (before the Loki Season 2 finale, apparently, though we're not sure how this retcon fits into the Disney+ series just yet).
Loki warns Steve about the TVA's plans to prune his new timeline, and offers to move them to a world where they can safely hide because they don't exist: Earth-828, the home of the Fantastic Four. Loki leaves the former Captain America with a card he can use to contact him, and he, Peggy, and Jim spend a happy 10 years there together.
The Fantastic Four: First Steps' post-credits scene is expanded on, and Doctor Doom tells Mister Fantastic about the Incursions and the fact that their universe is next. They figure out that someone is living in their reality who shouldn't be there, and visit Steve, who agrees to join them on a trip to Earth-616, where he successfully convinces his world's heroes to help Doom enact his plan.
Avengers vs. X-Men
The Avengers vs X-Men battle is a smaller part of the movie than you might expect, and the fight ensues when Earth-616's Mightiest Heroes arrive seeking help (though it's not specified in this leak, we'd assume they're there to learn more about Beast's device).
The mutant team has survived multiple Incursions and are unerstandably hostile. Later, we learn that Doctor Doom and Reed Richards have a plan to build massive cannons on the remaining Earths, which will somehow separate these parallel realities and end the Incursions.
They'll have to split into teams and visit different worlds to place them there. Meanwhile, Steve, now reunited with Thor, tells him he met a Loki who is still alive and agrees to take the God of Thunder to his brother in the TVA, which they locate with the card mentioned above.
Doctor Doom's Plan Revealed
When Steve and Thor get there, they find that everyone is dead and Loki has had his heart ripped out. Figuring out that Doctor Doom is responsible, Steve confronts him and learns the villain's true plan: to allow the Incursions to happen so he can rebuild the universe in his own image and resurrect his dead family.
Things get a little vague here, but Doom takes Steve to his castle in Latveria and introduces his masked followers, one of whom is supposedly impersonating Peggy to manipulate Cap. His presence on Earth-828 is what caused the impending Incursion in their reality, prompting Doom to conduct dangerous experiments that eventually went horribly wrong, killing his wife and child, and leaving him disfigured.
That's why he's now trapped in his armour and mask, and to make Steve pay, he kills Jim in front of him. As of now, we don't know whether Victor clashed with the Fantastic Four before or after this accident.
Final Battle In Latveria
It seems Thor has told his fellow heroes about Doom's true nature, as the Avengers, Fantastic Four, and X-Men arrive in Latveria ready to fight. The God of Thunder prays to Odin, who sends an army from Valhalla to join the battle, and war ensues as the Final Incursion begins.
There's no stopping that, and everything is destroyed...apart from Doom. When he stole Loki's heart, he stole his powers and now uses them to create Battleworld. Resurrecting the fallen, Doom erases their memories and makes it so that they only remember this new reality (where they have new lives in this false reality). With that, Doom sits on his throne as the God Emperor of Battleworld.
We still have some unanswered questions, and there are some plot holes. Did Doom killing Loki start the Incursions, and if he'd already stolen those powers, why did he need to formulate that plan with the Fantastic Four and team up with other heroes? Was he just distracting them in a bid to stop them from preventing the Final Incursion? We'll have to wait and see, for now.
However, we can tell you that there's no link between Doom and Iron Man, and perhaps unsurprisingly, no mention of Kang. We've also learned that Doom's villainous turn occurs during the fight between the Avengers and the X-Men, so there are some key details missing from this initial leak...