Starlight Glimmer vows to go back in time to save Twilight from being assassinated. Before all that, she explains what happened after Twilight reformed her, before Twilight was assassinated, and after the assassination.
This story is considered a flashback of what happened to Starlight Glimmer before she was reunited with Twilight Sparkle in twotonedearly's Equestrian Civil War: An Alternate Universe.