Accepting an invitation to a party hosted by Princess Celestia, Twilight Sparkle and Pinkie Pie hope to gain some new clients in the chance they might be offered new cases to work on. But just as the party is about to get under way in the Royal Gardens, a murder is committed in the castle library, and one of the party guests is found dead under mysterious circumstances. While solving the case, the detective duo must also try to figure out why the mysterious thief known only as "The Dragon" has never been heard of outside of Canterlot.