Rainbow Dash has been part of the SPS (Secret Pony Service) for over three years. She was sent to Ponyville to spy on one of the most dangerous of all criminals in Equestria. She had a nice cover going until she made one slip-up. Out of the intelligent Twilight Sparkle, understanding Fluttershy, rational Applejack and the sensible Rarity, who finds out? The hyperactive, (suspected) drug addict Pinkamena Diane Pie.
Pinkie Pie threatens to tell everyone her secret unless Rainbow brings her along on her search. Pinkie makes a big joke of the whole thing but soon the fun starts to die. The killer is relentless and is not against killing young party planners. Pinkie begins to see Rainbow in a new light as she realizes that she is not only the third most effective spy in all of Equestria, but the third most effective assassin.