On a regular day in Ponyville, Twilight begins to study simplifying spells. Luna visits, just to ensure that Ponyvillians still like her. Twilight moves on from simplifying spells and begins to work on astronomy. She studies the complexity of stars, and decides to try a simplifying spell on one, which, unbeknownst to her, is one of the stars that helped Nightmare Moon escape. When she tries the magic on the star, it's dark magic causes a Magical Reaction, something that's never happened before, but has been dreaded. Twilight and all of Ponyville are plunged into a time long, long, ago, when Ponyville was in it's most primary state. Luna tries to take up the job of raising the sun and moon, but is only successful with the moon.
Pinkie Pie tries to brighten everyones day, with hilarious attempts to make fire. Applejack searches all of Ponyville for AppleBloom and the crusaders, and Rainbowdash contains a secret that's about to burst. Fluttershy misses her animals, and upon discovering a dinosaur, she decides to raise it, with disastrous results. Rarity is devastated on the loss of her designs and fashions, and becomes obsessed with starting new trends. Finally, Spike runs away with the dinosaurs, and discovers some odd habits.

(Credit to Valkyrieskys on Deviantart b/c it is totally amazing!)