The Doctor, on the Master's half joking suggestion, takes Derpy as a companion in the TARDIS. He intends this as a relaxing break from his usual shenanigans, but he quickly finds himself trapped in an incomprehensibly large and complicated conflict millennia in the making. He has to escape, but how can do that when he has no idea who or what his enemy is, and the one who trapped him is himself?
Aside from knowing the TARDIS is a time machine, Doctor Who knowledge should not be necessary to understand the story, but you will get a lot more of the jokes.
Prequel to Obama Goes to Equestria.
Celestia, a nerdy unicorn, and Luna, a sporty pegasus, meet and sparks fly. Their friendship leads to the end of the world as they know it.
Derpy steps on a butterfly. This unleashes the most terrible evil the Doctor has faced and turns her into a sarcastic British theoretical physics professor.
You, a convicted serial rapist, are saved from the electric chair at the last minute by being teleported to Equestria. Sexytimes ensue.
Celestia makes a horrible mistake. She likes horrible mistakes.
Obama goes to Equestria and immediately plots to overthrow Celestia. This spawns a 10 story multiverse.
Shining Wings, a griffon, goes to Equestria to investigate a mysterious massacre. The only clue is a cryptic warning left by the last investigators: "Lavender Alicorn"
Maud Pie leads a geologic expedition to the lands outside of Equestria, which turns out to be a barren apocalyptic wasteland. But does she have an ulterior motive for this quest?