When Aryanne, Ponyville's resident Nazi sympathizer/unicorn supremacist (who is an earth pony), takes over as substitute teacher for Cheerilee's class, she discovers the ponies aren't quite as excited about eugenics as she is.
Trying his hand out in being a teacher, Discord starts his first lesson with Celestia. Time for the princess to learn how chaos theory really works. Hopefully things won't get too chaotic, or at least before the test.
After Celestia humiliates her at a dinner, Luna storms off and ends up snooping around in her sister's room. There, she stumbles upon the journal Celestia kept immediately after the banishment of Nightmare Moon.