This story takes place after the events of the My little pony movie (warning: spoilers for the movie is ahead!)
Princess Celestia and Princess Luna have been busy defending Equestria and dealing with their own issues for so long. After the Storm king destroyed the palace and it's being repaired the two decided for five days they want to explore the ocean as sea ponies. They'll spend the time relaxing and enjoying each others company while exploring a new world without having to worry about anything.
But during their getaway, the two princesses are swept out to sea by a un-natural storm. Now the two are lost in the vast ocean filled with strange creatures and new races! The two princesses will have to work together to get back to Equestria, or be fated to be stuck wondering the oceans as sea ponies forever!