On the Nightmare Night Canterlot castle is empty, since everyone went to celebrate. Except for two ponies, deeply attracted to each other. They play an alluring game, hoping to bring their relationship to next level.
(The teen rating is only for the tension, that follows characters throughout the fic, and shapes the style of the story accordingly.. There are some - not too subtle - hints, about things that happen prior to this story, and it's pretty obvious what happens after this story, but there is no actual sex in the story)