The “What If” series is an alternate take on the Marvel Universe where things happen slightly differently from the main reality.
It is based on the long-standing comic series of the same name which was Marvel’s response to DC’s Imaginary Stories which followed the same premise and was later on firmed up as the Elseworlds imprint.
The comics center around small changes to the various Marvel teams (FF consisting of Spider-man, Wolverine, Hulk, Ghost Rider), different backstories for their characters (Namor is raised by his human father on land), change in one specific event (Gwen Stacy’s death, Charles Xavier becoming Juggernaut instead of his step-bro Cain Marko) that has had major repercussions for Earth-616, or simply divergent narratives (Thor becoming Galactus’ Herald, Wolverine becoming the Punisher) and then allow the butterfly effect to do its job.
The animated series (which I do not have high hopes for seeing that Marvel animation is quite sub-par) has only revealed that they would be focusing on the overall MCU and how a small change could have resulted in a different arc altogether. The first episode would center around Peggy Carter taking the super-soldier serum instead of Steve Rogers.
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Some fans are speculating that it might have one of the most intriguing but frankly underwhelming comic stories, Marvel Zombies.