Mysterious Mare Do Well had embarrassed Rainbow Dash.
Rainbow Dash eventually caught the Mysterious Mare and revealed who it was, then it turned out it was the rest of the mane 6.
Pinkie Pie, who Rainbow Dash unmasked, did what Pinkie Pie does, to save some construction workers.
Twilight Sparkle used magic to fix a dam.
Fluttershy flew away (that's all she did in the outfit, making Rainbow Dash think this wasn't Twilight.)
Applejack used her leg strength to stop a carriage headed off of a cliff.
Rarity, all she did was design the costumes.

Rainbow Dash laughed after initially being sad about her friends doing this to her.

4 weeks later

Rainbow Dash sees Mare Do Well flying with the Wonderbolt Captain, SpitFire. She follows them to the Academy but she was caught by Soarin' who said that she wasn't allowed here and Rainbow knew that she could easily get in, but would rather not do something that could affect her chance at the Wonderbolts.

Rainbow Dash thought to herself she would get to the bottom of what was going on. She thought Fluttershy was pranking her (of all ponies) and decided to pay fluttershy a visit (this is where the story will start)