Celestia and Luna married the mane six, now they have a harem and a honeymoon to deal with. In all honesty, happily ever after isn't all it's cracked up to be. At least there'll be room for orgies.
Have you ever gotten so drunk you forgot you offered the princesses' hoof in marriage to whoever brought your sister back? And how many 'es' there are in 'Princess'?
Life after ruling for Celestia is boring, almost too boring. However, on one of her strolls, she comes across a lost runaway filly. It quickly becomes clear that there is more to the filly than what meets the eye.