On October 10 2010, Equestria appears in the middle of the Pacific ocean, from where it was originally stolen by Discord.
This has catastrophic consequences for the human race.
Most of them are fine.
Some of them die.
A fair number are twisted by chaos magic into freaks of nature.
A rare few gain magical powers, without being deformed.
And a very rare few turn into creatures from that world.

Now, Equestria and Earth have to deal with the repercussions of their reuniting.