Celestia asks Luna to teach Twilight dark magic. Twilight must pass Luna's class in order to graduate. As Luna develops feelings for her student, Twilight questions which princess she really wants to serve and protect.
Twilight Sparkle wakes up to a very unwelcome idea. Now she needs to find a way to clear it out of her mind. Because if she falls asleep before it's gone, the Princess of Dreams will get to see that idea for herself.