"What's Past Is Prologue" is the eighth episode of the fifth season of the American television series The Flash, based on the DC Comics character Barry Allen / Flash, a crime scene investigator who gains super-human speed, which he uses to fight criminals, including others who have also gained superhuman abilities. It is set in the Arrowverse, sharing continuity with the other television series of the universe, and is a spin-off of Arrow. The episode was written by Todd Helbing and Lauren Certo, directed by Tom Cavanagh, and is the series' 100th episode.