Twilight Sparkle is summoned to Princess Celestia's castle early one morning, via a letter stating that she is being held captive and needs Twilight's aid. However, things take a strange turn once she arrives.
(I wrote this for the April 2011 friendoff on Equestria Daily. It's not super-graphic, but I think it's enough that a teen rating sounds good. It's also slightly edited; I decided while rereading that I wasn't quite happy with one part in the middle)