Sunset Shimmer was able to grab an enchanted book before she left Equestria during her last visit, and a very racy one at that. As she was about to read it, Vice Principle Luna caught her and inadvertently activated it.
 
It’s been three hours so far but for both women it’s felt like they’ve been in the book for what feels like three days. In those three days the two have been made to sleep with each other and characters from the book.
 
Things will be so awkward when they get out.