What's Past Is Prologue (The Flash)

"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.

Grant Gustin stars as Barry, and is joined by principal cast members Candice Patton, Danielle Panabaker, Carlos Valdes, Hartley Sawyer, Danielle Nicolet, Jessica Parker Kennedy, Chris Klein, Cavanagh, and Jesse L. Martin. The episode sees Barry and his daughter Nora travel back in time and take a trip down memory lane where they cross paths with several of the show's past speedsters in order to gather some key necessities to stop Cicada. Several recurring guest stars make appearances, including Michelle Harrison, John Wesley Shipp, Teddy Sears, and Tony Todd. Shipp also reprises his role as Barry Allen / Flash from the 1990 television series in a pre-credits scene that ties-in with the Arrowverse crossover "Elseworlds".

"What's Past Is Prologue" originally aired on The CW on December 4, 2018, and was watched by 1.78 million viewers according to Nielsen Media Research. The episode received a positive critical response.