Sunset Shimmer visits Equestria with Juniper Montage. Soon, It's time for them to return to the human world -- followed by Pinkie Pie and Princess Luna, -- but it turns out that, at that moment, a certain hacker has glitched the system, and the four of them end up in an entirely different world.
An Overwatch/MLP crossover.
Haven't even read, but the fact you outright say this is a crossover means you should have used the crossover tag.