Twilight, Pinkie, AJ, and Rarity no longer remember anything about the Secret of the Pegasi, but there's one thing Rainbow and Fluttershy overlooked: how did the letter that Twilight wrote about the Secret get to Celestia?
Exactly.
When Twilight comes stumbling back home with a hangover and no memory of the previous day, Spike discovers that he's suddenly alone in knowing about the mystery of removable wings. An off-handed comment by the worn-out unicorn prompts Spike into an investigation that will shed more light on the truth behind the conspiracy and raise even more questions.