The planet of Aphrodite is a true paradise world filled with very friendly natives, an always pleasant atmosphere, and more attractions than anypony knows what to do with. But most famously is a strange magical radiation that covers the planet which, upon contact, will reverse a creature's gender so long as they're in contact with it. Unsurprisingly, this has made it the galaxy's most desired vacation spot for adults of almost any species. 

So, when the companions of the Doctor needed a break from their adventures through time and space, all but one chose Aphrodite as their destination. The stallion, Feather Wind, simply wanted to relax, read a book, or meditate on the nature of magic, not put on dresses as a mare. It didn’t help that he was small to begin with and his marefriend was a giant. So, when their roles in their relationship were reversed, there were going to be problems. 

Unfortunately, like all things with the Doctor involved, nothing is ever as simple as a vacation, and Feather finds “herself” in over her head in ways she could have never predicted.