After having a massive argument, during a extreme mood rollercoaster, with her husband Shining Armor, Cadence decides that the stallion needs to be taught a firsthand lesson on a mares feelings and needs. A quickly thrown together plan later and Twilight finds out she now has the little sister that she always wanted and Shining is now stuck as a filly till they decide otherwise. That is if they don't forget about him ever being a stallion first.