After a failed mission in the Multiverse, a biological virus from the dark dimension (Earth-9811) leaked to the main Earth (Earth-616). This virus does not kill people — it turns them into superhuman zombies: still intelligent, still powerful, but completely devoid of humanity.
After only 72 hours, the Avengers and X-Men were separated, half infected. Tony Stark – in a last ditch effort – sealed himself in the Mark-Z armor (Z = Zombie-Proof), becoming one of the leaders of the resistance. Spider-Man, devastated by having to confront his own family, leads a group of survivors including Shuri, Kate Bishop, Ghost Rider, and Deadpool (who is immune because he is… long dead).
Meanwhile, Captain America (zombie) and Scarlet Witch infected with the virus form an army of super-zombies, using tactics, strength, and trickery to take over major cities, turning the world into a wasteland.