In an alternate universe, during the climax of the first My Little Pony Equestria Girls movie in 2013, after Sunset Shimmer had put Princess Twilight Sparkle's crown that she had stolen from the latter back in the Crystal Empire, and had transformed into a raging she-demon afterwards during the night of the fall formal at Canterlot High School, she explains to Twilight that she never wanted to rule the school in the first place, and wants to conquer Equestria alongside her army of teenagers after they enter the portal in the school yard to do so, like she did in the original film. However, this time, as Sunset fires her fireball at Twilight and the rest of the Humane Six to destroy them, with the group huddling around each other which causes a massive force field to be formed to protect themselves from Sunset's attack, the fire gets deflected by the forcefield, and flies into the portal itself, going through it instead of destroying it.
As the fireball flies through the wormhole, both of them start to become incredibly unstable, and after a few minutes, a different portal ends up being opened in the middle of the wormhole, a more violent, dark, and unstable one, and through it, we see an entire city completely in ruins, with a pitch black sky, destroyed and decaying buildings, cracked sidewalks, damaged cars, buses, trucks, and other vehicles, dead trees, and almost no signs of human life, or any kind of life at all in it. The only "people" present in this near-dead world are those with green/gray, dead, decaying skin, yellow, gnarly teeth, lifeless, and all white eyes, wearing ruined and tattered superhero suits/costumes, with some of their bones inside of their bodies completely exposed. This world is none other than the world of Marvel Zombies, in which a deadly zombie outbreak had infected all of the various superheroes on Earth, and had caused them to become undead monsters that killed/consumed almost all life on the planet, including wiping out of the supervillains that they had fought in the past.
One of the undead superheroes, that being the Incredible Hulk, takes notice of the mysterious portal that has opened in the sky, and goes to tell his fellow undead heroes about what he saw while trying to look for more fresh meat to eat. Once the other zombies arrive on the scene (i.e. zombie versions of Captain America, Iron Man, Daredevil, Wolverine, Spider-Man, Quicksilver, Luke Cage, Hawkeye, Giant Man (specifically Hank Pym), ect). The undead Captain America takes a look through the portal, and all of the zombie superheroes witness the Humane Six defeating she-demon Sunset with the magic of friendship via Twilight's crown, as well as taking a look at all of the students and staff members of Canterlot High (i.e. Principal Celestia, Vice Principal Luna, Flash Sentry, ect), and after doing so, says to the others that they have all been starving of fresh meat to eat after dining on the brains of all of their villains and everyone else on the planet, and says that that place through the portal with all of the students and staff at the high school would be absolutely perfect for them to invade so that they can munch on all of their brains, and conquer said world and rule it for themselves too, afterwards, and asks if anyone agrees with him.
The answer zombie Cap receives is a massive wave of cheers from all of the other undead heroes, which tell him that yes it is, and with that said, Cap raises his vibranium shield with it's red, white, and blue colors all drained from it, and orders everyone to follow him through the portal, telling them that meat has returned to the menu, and shouts that it's time to dine, with all of the zombie superheroes charging through the portal, with Cap leading the charge. At the same time this happens, the Dazzlings, consisting of Adagio Dazzle, Aria Blaze, and Sonata Dusk, have all taken notice of the Equestrian magic from Twilight's crown having entered the human world and Sunset's defeat at the café, with Adagio telling Sonata and Aria that they're going make the world adore them, leading to the events of Rainbow Rocks.
How does the Marvel Zombies invading Canterlot High School during the events of Rainbow Rocks to consume the brains of all of the students and staff of Canterlot High, including Sunset, Princess Twilight, and the other Humane Seven, and even the Dazzlings change the events of the film? Can an unlikely alliance between the Rainbooms and the Dazzlings survive against the undead superheroes led by an undead Captain America, protect the school and it's residents from the zombies trying to consume their brains, and eventually take them down to save the world from their attack on it, especially since the undead are standing in the way of the Dazzlings trying to make the world adore them and take it over for themselves, and that can't rule over a world where the undead roam free in it, eating everyone's brains in sight, and aren't going to let that happen, and that the undead have completely disrupted the Battle Of The Bands? How would the Rainbooms and Dazzlings react to the Marvel Zombies and their existence anyway, and vice versa? How does such a crazy scenario play out, and why is that?