Luna is no longer Nightmare Moon, and has returned to rule Equestira once again, but not everypony is so ready to forgive. When a secret organization kidnaps the Elements of Harmony and threatens to murder Luna, Rainbow Dash must go back into the memories of her ancestor Firefly, to learn the ways of the Assassins to fight this new terror and save the new princess, as well as her friends.

Credit to he4rtofcourage for the title image.