Two agents--Bill Collins of the ATFE and Tina Mattson of the FBI--are assigned to investigate just what happened the night of the Fall Formal at Canterlot High. They're confronted with students claiming that one of them, Sunset Shimmer, became a demon and nearly destroyed the school, only to be stopped by one Twilight Sparkle. Yet there's no record of Twilight Sparkle's existence, and Sunset Shimmer herself seems to be an enigma. With Sunset Shimmer in custody on suspicion of terrorism, Collins and Mattson try and learn the truth...only to find that some cases must never be solved, and some secrets must never be spoken--including their own.

Takes place immediately after the events of Equestria Girls.