For the first time in several hundred years, Twilight Sparkle–Alicorn of Magic, Princess of Friendship, and sole Ruler of Equestria–has a friendship problem to solve.
Pinkie Pie is alone in a place she does not want to be. She wants to go home. She wants everything to return to normal. Pinkie Pie has to deal with the reality of a tragedy she does not want to face.
Twilight suspects something is going on when she starts forgetting large chunks of her days. When Spike reveals the answer, it's more than she wanted to know, and much more than she ever needed.