When River Song tried to rewrite a fixed point in time, time died. She and the Doctor fixed it, of course, but such events are not without consequences. The universe is no longer behaving the way it should. The time lock that holds Gallifrey and the rest of the Time War is weakening, and every universe is in danger.

Apparently somebody knows what to do about it. A mysterious voice sends the Doctor to Equestria as a pony to find the solution. Much to the Doctor's dismay, he has no idea what he's supposed to do. Can he save everything in existence - again?