Princess Cadence is a foalsitter who, being in rich, hoity-toity Canterlot, has to put up with spoiled brat after spoiled brat. Twilight Sparkle is a terrifying filly who drives away sitter after sitter. They were bound to find each other.
Following the events of "Three's a Crowd," Cadence tells Shining Armor about her time with Twilight and Discord over a crystal mud bath with her husband.
One fateful morning, Princess Luna finds Celestia has turned into a megalomaniacal tyrant with one clear goal in mind: to be sent to the moon. She even has her bags packed and everything.
General Iroh of the Fire Nation finds an infant Princess Cadence shortly after the death of his son. Taking it as a sign from the spirits, he decides to raise her.
Cadance didn't know what to expect when her formerly banished Aunt Luna returned from her thousand year imprisonment in the moon. But, she's quite entertained by the hilarity.
Cadence made a mistake and plans on fixing it. However, fixing this mistake will be difficult since she has to face being dethroned for what she wants.
For the past year, the four rulers have gathered in the unassuming town of Ponyville for meetings of the highest secrecy and order. If the ponies of Equestria knew what really happened behind these doors, they would probably switch to democracy.
The princesses gave their magic to Twilight to prevent Tirek from stealing it. As a result, they end up stuck in Tartarus. The fact that things manage to go downhill here says a lot about the wonders of family.