A human wakes up in Equestria with a splitting headache and a kind, concerned pony tending to him.
Unfortunately for the ponies, the human thinks he's dreaming.
Doubly unfortunately, he's a sadistic snuff freak who's been beating off to the thought of raping and murdering ponies for a decade, and he thinks this is a harmless, consequence-free place to indulge his fantasies.
We'll see how far into the main six ponies we get.