The first time around, Nightmare Moon's return nearly caused a national catastrophe and had to be resolved by the forging of friendship between six ponies who had only met that night.

Evidently not having learned anything from the experience, Nightmare Moon tried to do it again last month. Except Luna wasn't having any part of it this time. Instead, she's teamed up with Twilight Sparkle to make Nightmare Moon learn that revenge can be a dish that can taste far worse than just bittersweet.