Marvel Studios has released an official plot synopsis for Avengers: Doomsday, confirming the film will bring together heroes from "three distinct universes" on a "deadly collision course" as the MCU's Multiverse Saga reaches its conclusion. The description signals the vast scale of the crossover, which is expected to unite the core MCU cast with characters from previously separate franchises, though it stops short of naming the threat they will face.

The three universes are widely believed to be the main MCU (Earth-616), featuring both Sam Wilson's Avengers and the New Avengers alongside Thor and the Wakandan heroes; the Fantastic Four's Earth-828, who crossed over via Thunderbolts*/The New Avengers' post-credits scene; and the Fox-era X-Men universe, Earth-10005, including Professor X, Cyclops, Nightcrawler and Magneto. Speculation points to the "existential threat" being an Incursion event rather than Doctor Doom himself, while the synopsis notably omits any mention of Steve Rogers' current whereabouts, fuelling fan theories about his fate.