Twilight Sparkle is sent to stop a princess, (won't tell you who), who is threatening Equestria. Can Twilight stop her? Is Twilight's target even the threat? Is it somepony or something else?
So Celestia can explain the Discord situation in the middle of Canterlot, when Discord can literally appear anywhere, any time, but she has to knock out and kidnap Twilight to Zebrica (where she might be allergic to the air) to tell her that Luna might be stealing books?
Other than that point, I thought this was a good start. I thought the pacing was a tad quick, but a little more detail would fix that right up.